RT-PCR: Not seeing how it can measure mRNA expression levels

Question: From my understanding, in RT-PCR we start off with an mRNA molecule, use the enzyme reverse transcriptase to create a cDNA copy (hence creating a double-stranded mRNA / cDNA hybrid), degrade the mRNA, and finally create a complementary strand to the cDNA with a DNA polymerase. If we're ultimately creating DNA and it's being duplicated at each cycle, how are we able to deduce the original quantity of mRNA in our sample? It seems like we would need to combine it with some other method, but a lot of sources I'm seeing seem to imply that RT-PCR alone is enough to "quantify" the amount of mRNA in a sample. Any thoughts on this are welcome
Answer: There might be a confusion of terminology. There is real-time quantitative PCR, which can sometimes be called RT-PCR. There is also reverse-transcriptase PCR, which is also confusingly called RT-PCR. To try to reduce confusion, real-time quantitative PCR is also called RT-qPCR or qPCR.
What you describe in your question is the reverse-transcriptase type of PCR. This is used for amplification, not quantification, of starting material.
QPCR or quantitative PCR, on the other hand, uses fluorescent dyes to measure the amount of amplified gene product. Standards provide a baseline measure of how much genetic material you should expect to see over time, at each PCR cycle, for a given amount of measured fluorescence. Measuring fluorescence of your own sample over time, and comparing it against the standard, gives you an estimate of what you started with, initially.
Answer: I just wanted to elaborate on the previous answer regarding the confusion between real-time (quantitative) and reverse-transcription(RT) PCR. RT-qPCR refers to real-time quantitative PCR, but some organizations refrain from any reference to quantitation (because it's really only semi-quantitative), instead using the nomenclature rRT-PCR (reverse transcription real-time PCR, I guess). In either case, this technique is theoretically capable of quantification of mRNA copies using the appropriate assay design and standards.
The general principle is that, after the cDNA is reverse transcribed from RNA, it is amplified similar to a normal PCR reaction. This can be accomplished in a single reaction with master mix containing both the reverse transcription and qPCR reagents, or in two separate reactions, by first performing an RT first, and then a standard qPCR on the resulting cDNA. The all in one method requires fewer reagents and less hands on time, but it can be difficult to optimize both reactions at the same time.
Here's a good primer on qPCR assay design. As stated in the previous answer, the quantification comes from some measure of fluorescence related to the amount of amplification product present. In an optimized reaction, the number of amplicons should double with each cycle, allowing for quantification based on how many cycles it occur before a specified fluorescence threshold is reached by each reaction well. Intercalating dyes like SyberGreen quantify total DNA present, whereas fluorescent probes are more specific to your gene of interest. Because it's only semi-quantitative, a standard dilution series of known concentration is included on each plate and used for quantification. The standards should be of the same material and sequence as your gene of interest (DNA or RNA) and should be diluted into a range of concentrations that will likely include the concentration in your experimental samples. You also need to be careful about the possibility of degredation of your standards, as the quality of your quantification is entirely dependent on the quality of your standards.
An alternative approach is to quantify a gene of interest relative to some reference gene that is constituatively expressed at the same level regardless of the experimental sample. This method (sometimes called the delta-delta Ct method) eliminates the need for running a standard curve, but requires additional reactions or multiplexing, in addition to identifying a stable reference gene with primers and possibly probes that will work under the same thermocycling conditions as your gene of interest.
A third option is absolute quantification via digital PCR (dPCR). One issue with qPCR in general is that, even when running samples in duplicate or triplicate, it can be hard to confidently resolve differences in quantification smaller than about 3-fold. Enter absolute quantification. dPCR works by dividing up the reaction into tens of thousands of nano-liter sized compartments, such that each compartment may or may not contain an actual copy of your gene of interest. Fluorescence is only used at the end of the reaction to determine whether each compartment has amplified or not (hence digital, each compartment is a 1 or a 0). Quantification is calculated by determining the ratio of positive to negative compartments and comparing to a Poisson distribution for discrete probability distrubutions. It can have much finer resolution than qPCR, supposedly capable of reliably detecting a 1.2-fold difference in concentrations, with the trade-off that it has less dynamic range with reduced confidence in quantification estimates near the top and bottom of that range. Plus the entire system is usually much more expensive at just about every level (equipment, reagents, and consumables). But with most gene expression assays, you really need to ask yourself if a 1.2-fold change in transcript level is even going to be meaningful in your biological system.


How can I use date and time selectors in OneNote 2013?

Question: The default note template in OneNote 2013 has date and time selectors underneath the note title, which automatically inputs the date and time when the note is created and can be manually adjusted by clicking on the calendar/ clock icons.
How can I use these date and time selectors elsewhere in my notes? And can I attach them to note containers?
I am aware that date and time can be inserted from the Insert ribbon, but these are not what I am looking for.
Answer: You can't I'm afraid, it isn't available as data. OneNote isn't an information management tool as such, just a notebook.


Manifolds with homeomorphic interiors

Question: Suppose that two compact topological manifolds with boundary have homeomorphic interiors. Can we conclude that the two manifolds are homeomorphic? What happens in the smooth category?
Answer: In "Uniqueness of the Open Cone Neighborhood" by K.W. Kwun, Proc. Amer. Math. Soc. 15 (1964) pp.476-479, it is shown that if two manifolds have homeomorphic interiors then their boundaries have homeomorphic suspensions, or in other words the boundaries are h-cobordant. I don't know if one can conclude anything more in this generality.
Comment: I don't think one can say any more generally: If $W : M \leadsto N$ is an $h$-cobordism then $S^1 \times W$ is an $s$-cobordism (crossing with $S^1$ kills Whitehead torsion), and taking covering space corresponding to the subgroup $\pi_1(W) \leq \pi_1(S^1 \times W)$ shows that $\mathbb{R} \times M \cong \mathbb{R} \times N$. In particular, $[0,1] \times M$ and $[0,1] \times N$ have isomorphic interiors.
Comment: @Oscar. The conclusion that the two boundaries times $R$ are homeomorphic is the theorem of Kwun paper cited by Gjergji.
Answer: Gjergji Zaimi's answer gives a strong positive conclusion: the product of the boundaries with $\mathbb{R}$ are necessarily homeomorphic. I just want to add a couple of explicit examples illustrating that the boundaries themselves need not be homeomorphic.
The first example is given by Milnor in his article "Two complexes which are homeomorphic but combinatorially distinct", partially stated there as theorem 3. Consider the 3-dimensional lens spaces $L_1=L(7,1)$ and $L_2=L(7,2)$. In that article, Milnor proves (section 2) that for any $n>1$, the interior of $L_1\times D^{2n+1}$ is diffeomorphic to the interior of $L_2\times D^{2n+1}$. However, Milnor also proves (at the end of section 4) that their boundaries, $L_1\times S^{2n}$ and $L_2\times S^{2n}$, have different Reidemeister torsions. Consequently, the boundaries cannot be homeomorphic, by the topological invariance of Reidemeister torsion.
To obtain other examples, we can apply the result stated by Oscar Randall-Williams' in his comment to Gjergji Zaimi's answer. It suffices to find $h$-cobordant closed manifolds $M$, $N$ of dimension greater than 3 which are not homeomorphic: then $M\times I$, $N\times I$ will have diffeomorphic interiors by Oscar's argument, but their boundaries are not homeomorphic. As Milnor proves in the article cited above, we can take $M=L_1\times S^{2n}$ and $N=L_2\times S^{2n}$ for $n>0$. I wrote a more detailed discussion concerning examples of this type in an answer to a related mathoverflow question.
Comment: Thank you Ricardo, this is a very interesting  answer, I didn't know this result of Milnor.
Comment: @Daniele: You are most welcome. I am glad I could help.

Manifolds with homeomorphic interiors


120V outlet from 240V load center?

Question: I want to install a 120V outlet for my deck, and I'm not sure if I can run this from the same load center that my hot tub uses. It's a 240V load center that is fed from a double pole breaker in the main panel inside. If I can, I would much rather run the outlet from this panel instead of running it from the one in my garage and digging a trench to the deck from there.
Comment: Pictures of what you have for the load center you want to add an outlet from would be helpful.
Comment: Can you post photos of the loadcenter/breaker panel, yes? (Preferably including at least one showing the entire interior of the panel, with the feeder breaker off, of course)
Comment: @Ecnerwal Here's the load center outside
Answer: Get that bond screw pulled, and get a PK7GTA in there, then you can add a circuit here
The first step is to sort out this morass of a spa panel box.  The good news is that it's a QO612L100RB, which means that you can get just about any sort of breaker you'd ever want to put in there.  The bad news is that the original installer butchered the neutral/ground handling, leaving the bond screw in and spamming all the ground and neutral wires onto one bar, so we have to fix that before we can start adding circuits.
You'll need a PK7GTA ground bar and a screwdriver to fix this, in addition to the breaker for the deck outlet.  First, turn off power at the feeder breaker for this panel.  Second, you'll need to remove the green screw next to the neutral lug; this removes the bonding screw from the panel, and separates ground from neutral here.  Third, you'll need to install the PK7GTA in the panel, screwing it to the raised holes bottom left or bottom right.  Finally, you'll need to take the two bare ground wires out of the neutral bar and connect them to the new ground bar instead.
Once this is done, you can add your branch circuit as normal. The decision as to whether to use a GFCI receptacle with a regular breaker or a GFCI breaker with regular receptacles is yours; the NEMA 3R enclosure of the panel might provide marginally better protection for a GFCI than a receptacle box would, but GFCI breakers are much more expensive than even a weather-resistant GFCI receptacle, so there is that as well.  Don't forget to size the breaker appropriately for the wiring you are running for the new circuit (15A for 14AWG, 20A for 12AWG), use properly outdoor-rated wire (UF instead of NM, or THWN in conduit if you are going the conduit route) and boxes (with while-in-use covers), and torque all breaker and loadcenter screws/lugs to specification with an inch-pound torque screwdriver (as per 2017 NEC 110.14(D))!
Comment: Good observation .  My eye was looking at the breaker,  was wondering if hot tub has gfci  protection. Good time to fix that .And do the outlets have to be away from hot tub 5 or 6 feet?
Comment: Thanks a lot for the advice, I'll be sure to get that fixed
Answer: Great - looks like you have 4 double (240V) spaces, with 3 of those free, so you have space for as many as 6 single (120V) breakers.
How much spare capacity you have, if any, will depend on the feed breaker in the main panel, and the actual load on your 50A hot tub breaker.
That is, if the feed is also a 50A breaker, your 120V load would have to be less than the difference between 50A and the actual load of the hot tub, or you risk having the feed breaker trip. Please note that you don't need (say) a 70A feed to have a 50A and a 20A at the sub-panel - but if they were fully loaded, you would. Note that (were that the case) you could have 2 20A (120V) breakers without needing any more than 70A maximum feed, so long as they were on opposite legs (generally for this type of panel, side by side) since the 20A on one "leg" balances the 20A on the other "leg", so 50 + 20 + 20 only comes to 70 if the 20's are properly arranged 120V 20s.
If your anticipated deck outlet loads are small, you probably have nothing to worry about. If they are larger, you might need to upgrade the feed breaker and perhaps also the wire from the feed breaker to the sub-panel.
You may also have a problem in that I think it looks like you may NOT have ground and neutral isolated in this subpanel, which they definitely should be.
Comment: Ok thanks for the help. The feed to that box is a double pole breaker (says 50A on each switch), so will this be ok to add just one breaker? I only plan to add one 15A or 20A breaker for a single GFCI outlet which will be used mostly for some decorative lights, so it won't be a large load.

120V outlet from 240V load center?


Possible to charge device with Google Pixel phone through USB-C?

Question: Is it possible to charge an external device using Google Pixel smartphone as a battery? In other words, can I change the charging direction (reverse charge) so I can charge e.g. a laptop using my Google Pixel as power source?
Answer:
Pixel phones support USB C Power Delivery
This standard says

Power direction is no longer fixed. This enables the product with the power (Host or Peripheral) to provide the power

Which translates to reverse charging

Reddit users confirm reverse charging with Pixel

How efficient charging your laptop would be dependent on the state of charge of phone / laptop , battery capacity of laptop but theoretically yes, you can ( you would need USB cable with male connections at both ends)


iMessage issue when being sent to cell number

Question: A friend of mine has iPhone 4S with iOS 5.0. I can iMessage him using his apple id fine. However, when I try to use his cellphone number, I never get the light blue send button. It stays green all the time. This seems very strange to me. Any explanations?
Comment: Does your friend use a service like Google Voice?
Comment: Might it be that the cell number is forcing cellular texting and the Apple ID is allowing him to get the message via wifi if he's using a wifi network. Remember, iMessage is still in beta so this may not be the way it's going to work when released.
Answer: iMessage sends a text message to get activated on the first boot of the phone. Maybe this text message didn't arrived at Apple, so that his iMessage is only activated for use with his Apple ID.
He could try to deactivate and reactive iMessage.
Comment: Deactivated iMessage. Restarted iphone. Activated iPhone and it worked liked a charm. Thanks Pre!


Can I apply for new visitor's visa while current one is still valid?

Question: I have a Canadian Visitor's Visa that expires in June. I want to travel to Canada in July or August. Can I apply for a new visitor's visa while the current one is still valid?
Answer: Yes, of course you can apply for a new visa, as the validity dates do not overlap.
You should check your passport validity before applying. Canada no longer issues visas that expire after the passport expiration date. You may need to renew your passport first.


Recover photos and videos from s3 (no sd or root)

Question: Basically someone deleted photos from my dads phone. He doesn't have a sd card and his Galaxy S3 is not rooted. Is there any way to get them back?
Answer: I am not sure if you can recover data from internal memory, normal recovery softwares require the device to be in USB/ Mass Storage Mode for it to detect the device to scan.
There is one method but it requires the phone to be rooted and it's a long process. If you are interested here is the link to it
Comment: Won't rooting it make the phone lose all the data?
Answer: Depending on the mobile carrier, the phone may have pre-installed software that has been backing up various files to their servers.  For example, my S3 from Verizon has something called Backup Assisant Plus that came with the phone and was on by default.  I would check the settings in the phone and the carrier to see if the phone was sold with a default backup system.


Confusion about calculating inductance of a coil

Question: I'm a little confused about how to calculate the inductance of a wire coil.  While I thought this would be really clear, what is confusing me is that there seems to be different ways to calculate it.  In this question, the result of the inductance formula is dependent on cross sectional area, number of turns, permeability and the length of the coil.  For the calculator on EEWeb, the inputs are the diameter (and hence the cross sectional area), the number of turns, permeability and the wire diameter, and the wire diameter, rather than the length of the coil.  I'm unclear how the formulas are related or which one to use.  My guess is that the 2nd calculation determines the inductance for a coil one meter long and I have to divide by the length of the coil I'd want to find the inductance of, but I'm not sure.
Comment: I believe the former formula is assuming the wire diameter is negligible. The later probably assumes a particular geometry so the length is dependent on the number of turns and the two diameters. Different assumptions were made is all.
Comment: Wire diameter is the same as wire gauge, which *does* affect the Q of the inductor, or its efficiency as an inductor. Q = L/R, so fine gauge wire will get you the same inductance as heavy gauge wire, but its DC resistance will affect RF performance.
Answer:
My guess is that the 2nd calculation determines the inductance for a
  coil one meter long and I have to divide by the length of the coil I'd
  want to find the inductance of, but I'm not sure.

No, it's got nothing to do with a 1 metre length of solenoid.
The EEweb calculator is definitely problematic because it assumes all the "turns" are stacked on top of each other and not laid out like a solenoid. This can be easily proven. For instance if I set up the loop diameter to be 279 mm and the wire diameter 1mm, I get an inductance of 1 uH.
If I double the turns I get 4 uH. If I make the turns 100, I get 10 mH - this is exactly how inductance would change if wound on a core with no leakage flux i.e. inductance is proportional to turns squared.

However, 100 turns cannot occupy a close-knit volume of space and at all possibly hope to have 100% coupling from every turn to each other turn.
Basically, it's a pile of doo doo.
Answer: Inductance calculations are very sensitive to winding geometry and scholarly correction factors exist. This is because there many more variables from simple designs to complex.
The basic calculation must include wire diameter, length and average winding diameter as fill factor , insulation and overlap have some variations.
The effective diameter Deff obtained from here is used to calculate the inductance of the single-layer helical coil:
where:
μ: permeability of the coil core
N: number of turns
l: length of the coil, measured from the connecting wires centre to centre
kL: field non-uniformity correction factor according to Lundin
ks: round wire self-inductance correction factor according to Rosa
km: round wire mutual-inductance correction factor according to Grover and Knight


Can I get the same results with Nikon D lenses as with G lenses?

Question: I have been searching for a while now and I came to the conclusion that when you fix the aperture ring of Nikon 1.4D lens on lowest stop, then we can change the aperture using command dial, plus the 1.4D version also provides the auto focus on bodies with a built-in AF motor. Why then buy the 50mm 1.4G or 1.8G instead of 1.4D or 1.8D?
Answer: The main reason someone would choose a G version of the 50/1.4 over the D version is if you're shooting a D3x00/D40 or D5x00/D60 body, which doesn't have a focus motor in it. On those bodies, the AF D versions do not autofocus, while the AF-S G versions do.
There is also the fact that G versions are typically much newer optical designs, with increased performance. A lot of the G versions are digital-era designs, while some of the D versions are film-era.  Sharpness and maximum aperture aren't everything when it comes to lens performance. The G version of the 50/1.4 looks to have better contrast and CA control than the D version when wide open (see: the-digital-picture side-by-side).
Answer: The two Nikon G lenses mentioned in the question are AF-S lenses that have an internal focus motor and can be used with any modern Nikon DSLR with auto focus fully functional. The two Nikon D lenses mentioned in the question are AF lenses that do not have an internal focus motor and must be used with a camera body that includes a focus motor in the body via a mechanical linkage if autofocus is desired.
The D3xxx and D5xxx series bodies (as well as some, but not all of the older Dxx bodies) do not have an focus motor in the body and require an AF-S lens for autofocus. The AF Nikkor D lenses must be focused manually when used on a non-motorized body.
Even when using a body that does include a focus motor, the Silent Wave motors in most AF-S lenses are faster and quieter than the camera's focus motor and screw drive linkage. The optics in the G series of lenses are newer and generally better than the optics in most of the corresponding D series lens.
For more regarding the differences between these two particular 50mm f/1.8 lenses, please see What's the difference between using a 50mm f/1.8G and a 50mm f/1.8D with a Nikon D80?
Comment: This is actually incorrect.  AF-S indicates the presence of an internal motor.  G indicates that the aperture is controlled by the body and lack an aperture ring whereas D lenses have one.
Comment: @JamesSnell Technically all G lenses are also D lenses (because they communicate distance information to compatible bodies), and Nikon seems to always have a lens or two that defies their own naming conventions. The vast majority of G lenses are also AF-S lenses. Most D lenses are also AF lenses. There are some D lenses that do not have an aperture ring. http://www.nikonusa.com/en/learn-and-explore/article/go35b5yp/which-nikkor-lens-type-is-right-for-your-dslr.html
Comment: With regard to the four lenses mentioned in the question the answer is correct. The 50mm f/1.4G and 50mm f/1.8G are AF-S lenses with internal focus motors. The 50mm f/1.4D and 50mm f/1.8D are both AF lenses without internal focus motors that require a body with a built-in focus motor in order to autofocus.
Comment: @JamesSnell The answer was also edited to clarify the issues you identified.
Answer: When comparing G and D lenses you have to do it on an individual lens basis and not a generalisation regarding each type. e.g. some G lenses do not have a built in autofocus motor and some D lenses do. G lenses include the D distance information. G lenses do not have an aperture ring.
When Nikon changes the designation of a lens sometimes it is improved and sometimes it isn't. Sometimes the autofocus is changed, sometimes the optics are and other times neither.
AF-I and AF-S lenses have an autofocus motor in them whether they G or D. e.g. the AF-S Nikkor 28-70 mm f/2.8 D lens has a motor. The Nikon 28-200mm f/3.5-5.6 AF-G does not have a motor.
Sometimes a D lens can have a better build quality.
Comment: All of your examples are rare exceptions to the general rules of Nikon naming conventions. The vast majority of "D" lenses have no in-lens motor. The vast majority of "G" lenses do have an in-lens motor.


Database updating in background is problematic

Question: I have to store some data into database after downloading it from server,I have succesfully downloaded the data ,but when i go to update it into sqlite database it takes time. In that case if user keeps the app is background and after sometimes user brings back the app in foreground then after few minutes it got crashed.
When user brings back the app in foreground then it takes high time to load the same screen where user has left it,and before the screen appears app got crashed.
Please help me on this....how can I keep this process running in background as well as foreground without any issues.
Comment: from time to time ... perhaps.. you should "reply" something.. maybe you can "accept" one answer..
Answer: Try this:
[self performSelectorInBackground:@selector(dataBaseUpdateMethod:)
                       withObject:dictForUpdatingTheDatabase];

Where, dataBaseUpdateMethod will be the method which updates the database & dictForUpdatingTheDatabase is the NSDictionary Object (or say any other object of any type).
Hope this helps.
Answer: update your sqlite database in a background thread (using performSelectorInBackground).
if you are updating it in main thread in that case when user trying to app in foreground then application main thread is busy to complete your update DB task and not able to load the application and cause a crash.
Answer: Use This:
[self performSelectorInBackground:@selector(updateDB:) withObject:dictionary];

Comment: Thanks ,but it didn't helped.


Cathode ray discharge tube

Question: I am learning about 'cathode ray discharge tube'. I know when electrons are given out from cathode, they travel to anode, because anode being positively charged attract those electrons and continue to accelerate those electrons. When the electron hit any of the gas atom in tube, it helps to knock out electron from it and therefore, electron knocked also start moving towards anode. But i think that one stage would come when there are no electrons left with gas atoms those can be knocked out and when electron hits the gas ions, it would just slow down the electrons(emitted by cathode) and eventually leading to no ray at the end.
Comment: Electrons do recombine with positive ions rather quickly. Somewhat similar scenario to the one you've described occurs in [cathode dark space](https://en.wikipedia.org/wiki/Glow_discharge#Cathode_dark_space). Note though that stripping off *all electrons* (and often even more than just a few) requires tremendous energies and extreme conditions, e.g. deep inside stellar matter.
Comment: A CRT is nowhere near 1E-14 torr. In the 1E-5 or 1E-6 perhaps if lucky.
Comment: @andselisk. or just a *very* good vacuum, with just a few hydrogen atoms in the CRT, perhaps <10^-14 torr. Not many atoms to ionize...
Comment: Why it involves knocking out and leading to chain. Why not the electrons travel directly from cathode to anode?
Comment: The behaviour of electrons in a cathode ray tube depends on the amount of gas in the tube. When there is lots of gas there are lots of collisions and what you observe is very different from when the tube is a hard vacuum and most electrons go straight from cathode to anode.
Answer: For a cathode ray tube (old-style TV), you want the electrons traveling collision-free in a straight line from the cathode to the screen (anode), where they generate light hitting a phosphorescent layer. For a neon light, you want the electron hitting gas particles so that the entire tube glows.
This is further explained here: Why does the cathode ray tube only start glowing at low pressures?
Comment: Cathode rays are electrons emitted from the cathode. The discharge is from gas collisions. In a TV tube, there's no gas involved, and the electrons zoom past the anode to hit the screen (they are then collected by other anodes). If they are in a tube with gas, they can hit gas molecules and light up the tube as they head toward the anode (no screen).
Comment: Then for cathode ray tube don't we need any gas? As the path of electrons would remain straight only if they don't hit other gas atoms. Suppose after hitting too they remained in the straight path, then what's the use of gas here as the electrons can directly reach from one end (cathode) to the other end (anode).

Cathode ray discharge tube


Gamerule specific to one person?

Question: I own a minecraft server, and I like playing on survival. Everyone knows I have been playing a while so they always try to kill me. Obviously I could ban these people, but I want to know if there is a way to change the gamerule keepInventory for only myself. It's a bukkit server, and I can implement plugins.
Comment: So I am guessing you can't use god mode since that would be "cheating" for you?
Answer: If it's a bukkit server, you could get multiverse inventories and only enable it for you. Other than that there isn't really a "single player" gamerule.

Gamerule specific to one person?


How to address a wobbly cassette

Question: I've had this mountain bike a little over 2 yrs. I had it serviced when I bought it because it was recommended even though it's not an expensive bike.  One day the chain slipped off multiple times which had never happened. Once I flipped it over I noticed the cassette was wobbly. Would this be an expensive repair and what exactly should I be looking at to have replaced?
Comment: based in the description, the best advice you can hope for is "take it back to the shop and have them look at it"
Comment: You need to tighten either the lockring or freehub. If you can find a bike coop in your area, they will give you access to tools and expertise. This isn't an expensive repair and most mechanics (at non-peak hours) will let you watch and explain what they are doing.
Comment: See if there isn't a "bike coop" in your town.
Comment: I suspect there's more than one thing going on here.  Please check if the rim moves side-to-side, to see if there's play in the bearings or if you have a broken axle.
Comment: Was the bike used when you got it?  Do you know what the mileage on the chain/cassette is ?  The chain slip may be caused by wear, which is different to the cassette's wobble.
Answer: Could be a number of things:
Cassette is loose on the freehub body. If the cassette lockring has come loose the cassette sprockets can slide axially on the freehub body and possible 'rock' on it a little. If you take the wheel out of the frame you should be able to see of the lockring is loose quite easily. To tighten it you need a chainwhip, lockring tool and a big wrench. It would not cost much to have local bike repair shop do this.
Freehub body is loose on the hub: If it looks like the cassette is firmly on the freehub body, but is moving relative to the rest of the hub, the body may be loose, or one of the bearings supporting it is damaged. To tighten the freehub you'd need to get the cassette off, and then figure out how your particular freehub is secured. Again, your local bie repair shop can do this for you.
Possibly the axle or hub bearings are broken. With the wheel in the bike try pushing and pulling laterally on the wheel rim. If there damage to the bearings there will be movement.
If the freehub or hub bearings are damaged on an inexpensive bike the best thing to do is just get a whole new wheel. Inexpensive wheels are not worth rebuilding to replace the hub as the cost of rebuilding will be a significant fraction of the cost of a new wheel.
Comment: Minor point- to tighten a cassette lock ring you don't need a chain whip. That's only for loosening it. And the wrench doesn't need to be particularly large. Any medium sized adjustable wrench should be fine
Comment: Thanks for ALL the feedback... will try ALL the suggestions asap and  hopefully be able to get back to riding sooner than later. Thanks again.
Answer: If it is a cheap screw on cassette, and the play is in the cassette itself,  just replace it. A lot of the cheap ones get play in them after a while. Cheers
Comment: what you are talking about is called a freewheel.
Comment: How can a beginner work out if they have a cheap screw on cassette?
Comment: I guess by looking at it, and seeing if it has an external lock-ring, Is the easiest way

How to address a wobbly cassette


How can I open .jar for modding?

Question: When I go to C:\Users\(user)\AppData\Roaming\.minecraft\versions\1.6.2, I see my .jar file but when I try to open it, it does nothing. So how do I open my .jar file so I can mod it? I also know that if I extract the files my minecraft crashes. Why is that? How can I open it so I can mod it?
Comment: @5pike Not always possible. Most jar files do not have editable code in it, but precompiled one to save you time when compiling your project. Then there is again the difference between access to the jar file's functions and methods and a pure executable jar file. To Raymond Green: I think most people take the byte code which is interpreted by the Java virtual machine and from there on decompile the jar file into somewhat readable code. Problem beeing that classes,variables, etc are not named as in the original code. This is just an educated guess though, so I'm not posting this as an answer.
Comment: How do you open it? Do you double-click it, or right click > Open with > Editor?
Comment: Also, I think this question might be better answered by gamedev.stackexchange.com as it deals with a game-specific programming issue.
Comment: @strike No, they're not trying to create mods (you can't do that by opening the jar). They're trying to install mods as an end user.
Answer: you use any package manager that can handle a zip, because a jar is a zip with a different extension, the jar is signed though so you'll need to remove the files in META-INF when you repackage
however I suggest you look at forge; this is an api to make modding easier and allows compatibility between mods
Comment: [7-zip](http://www.7-zip.org/) is a good one.
Comment: @Dillmo Or WinRar.

How can I open .jar for modding?


How is EC2 instance utilization computed?

Question: I'm currently on free tier but I'm thinking about moving into a t2.small instance to be able to do more. At this point, I'm just really interested in having some personal tools on the cloud (something that I could have done if I had spare machine that I can set up as a home server), so I'd like the cost to be very minimal. I checked the AWS calculator, and it wants me to estimate the utilization, but I don't really understand how it is computed. If your EC2 instance runs 24/7, does that mean it has 100% utilization even if it only does significant work for maybe some hours?
Answer: Utilization in EC2 for cost purposes means the number of hours in a month that the instance is up and running, compared to the number of hours in a month.
The workload is not a factor in price.
Instances in the t2 class also have a concept of "CPU Credits," because t2 machines are burstable.  They earn credits at a fixed rate while running,  spend credits at a rate that varies with CPU utilization, and can only use 100% CPU when there are sufficient credits on hand (1 credit = 1 core used at 100% for 1 minute, or 1 core used at only 10% for 10 minutes, etc.), but this also is not a factor in cost.  The credits are included with the instance, and the charge is the same whether they are used or not.  There is no way to purchase more credits, and the performance of the instance does not vary unless the balance is depleted, and will recover in its own whenever the instance is idle.
Comment: So that means if I have an instance up 24/7 for the entire month, it's 100% utilization? Is there a way to schedule when they start and stop automatically?
Comment: There are ways to start/stop on schedules and in response to monitored conditions, but this can also have the opposite effect if done in a cavalier fashion -- a machine stopped and started every 30 minutes is actually 200% utilization.  A start event bills you for the one hour minimum, even if you stop it almost immediately, which is not unreasonable.  Two "start" events per hour would then result in a monthly cost double that of a machine left alone.  This is not a secret, it's documented, but there are a lot of docs to familiarize yourself with.


How do I manage/oversee multiple agile teams around the world?

Question: I have been tasked to manage multiple agile teams in different business areas and geographical regions (time zone problems). Has anyone got any experience with this? It seems like the job will be more of a traditional project management over seer position.
Answer: I've had 2 experiences in the same company with distributed teams:

In the first experience, I was Scrum master and I had 2 members abroad (with 8hrs difference). We just couldn't control how much time they were exactly working on the project and they were constantly pulled onto other projects of the remote location. It didn't work smoothly.
In the second experience, I was a developer among a 20-people local team and 6-people remote team (4hrs offset). They had their own component and Product Manager. We sent 2 people over and changed them every month until they mastered the art. It worked great. We had a very defined trust for them and they had their autonomy.

For example, Agile explicitly says team should be colocated. If you have to distribute teams remotely, do attempt to give them as much autonomy as possible.
I would recommend reading the "How We Do It" of StackOverflow's blog, since StackOverflow is well known for being a remote-workers company. Zappier is also communicative about managing remote teams. I recommend Atlassian tools. Common traits are:

A persistent video chat
RapidBoards / Trello / any visual ticketing system
Emails
A team collaboration tools to write and talk: Google Docs, Confluence, CampFire...
I would add a good code management system: Stash + Bamboo (or any good code review + git + continuous build system).

Disclaimer: I used to work at Atlassian.
On the cultural side:

Hire people you trust,
Hire people who can write,
Arrange real-life gatherings from time to time,
Develop a strong culture about delivery and expectations.

The two linked blogposts are really interesting about culture.
Comment: So in the first case, the main problem was that those 2 members weren't fully dedicated to the team, not the geographical separation per se, isn't it?
Comment: Right, but there's no way to have control on this. You have to 1. trust them and 2. give them an incentive to complete their work. Autonomy and a local PM are one kind of incentive.
Comment: Thanks for the info and it is useful. I have an additional challenge. It is with a company and the teams aren't all from the same company. I have a mix of inhouse, independent contractors, vendor consultants and a few "big 5" consultants as well. I am hoping that they'll all fall in to the communication system but if they don't I'll have to look at something else outside agile. My worry is loyalty and agenda. I've seen bad things happen before when externals do what their side tells them rather than look after their client's best interests.
Answer: Communication is key
I was the Scrum Master for 2 teams in Hyderabad, India (contractor), one in Montevideo, Uruguay (contractor) and one in Washington DC for a previous company. The time difference with Hyderabad is 9.5/10.5 hours. Here are some best practices that we developed over a period. You may not be able to implement all these. Strive to get there or mitigate them where possible:

Face-to-face contact: Arrange for the overseas team members to travel to your location and vice versa. It is harder to develop good communication, collaboration and team spirit between people who have never met.
Overseas team should be able to operate fairly independently: To be effective the overseas team should have all the skills/roles to be able to operate fairly independently.

Initially we had developers and testers in Hyderabad and the designers in DC. Web front-end development requires close coordination between the dev team and designers. We could see the improvement when we hired a local designer.
We trained a local Scrum Master in Hyderabad and arranged for local Scrum stand ups. This improved team coordination overall.
We could see the limitations of not having a local Product Owner.

Arrange for more time overlap: Some of our architects used to log in at night to answer questions from Hyderabad. The Hyderabad dev team members used to come in late and leave late to have better overlap with DC.
Virtual work-spaces: Use web based tools to make story/task status, shared calendars, holiday schedules, contact info... visible to the entire team. Ideally the tool should allow threaded discussion in the story/task. If you try to do this using email, people will be buried in emails and things will fall through the cracks causing much frustration.
Get communication channels as close to face-to-face as possible: Group video calls for the daily Scrum stand ups, ability to share screens/whiteboards, team members logging into chat when at work are some things you can try.
Answer: A few things I learned about working with a remote development team by being part of one:

Transparency is the best policy
For every new mobile app project we take up, we work up a document containing all of the info we have on the client's product and business. This way, all of the requirements are clear to both us and the client, while we can always get back to this document to clarify things in the future.

A product specifications document consists of, but is not limited to:

Project overview
Project objectives
Technology used for the project
Project architecture
User flows
User stories (which include acceptance criteria for every user story)
Wireframes

We work within the scrum framework
Some of the tools we use:

Trello
Jira
Slack
Skype
and BitBucket and GitHub as code repositories (we're a mobile app development agency)

The Lean Canvas of the client's business can come in handy. The Lean Canvas, proposed by Ash Maurya in his book Running Lean, is a business model hypothesis testing and validation tool. It's also a simple, yet powerful tool, which can be used by the remote team to get a lot of valuable info about your product and your business, at a single glance. A user persona sheet can also be of use.

Leading a remote development team doesn't mean micro-managing the hell out of those people. Your collaboration with a remote team will be a lot more successful if you lead those people by inspiring them. Be there to answer their questions, be it product or business wise, be open about the strengths and weaknesses of the project; be clear about the expectations and the vision. Be like an open book, but also give your remote co-workers space to come up with solutions.


How do I restore my storage space?

Question: After download a big app,I it says that there is insufficient space; and it stopped downloading. After this i checked application manager and it said that I had low space despite the app not downloading what do I do?
Answer: That's a very loose question. If your phone comes with an inbuilt File Manager, use that to navigate to the location and delete and residual files.
You could also use third party applications such as Clean Master to detect and remove unwanted files.
Answer: Try deleting the cache in Settings -> Storage. It probably holds the downloaded part and it could be already large from earlier as it contains other apps caches.
Answer: Try uninstalling high mb apps that you don't need anymore,also try using SD Maid to remove useless files.

How do I restore my storage space?


How can we get less off topic question?

Question: 1 out of 4 question are being closed for being off topic. The majority of them are not closed by the community but by a moderator. Could the community be allowed to help close questions? Could a disclaimer be used instead of closing questions dealing with personal questions?
How can we encourage better quality questions?
Comment: I do not really understand how your title relates to the body of the question. Please clarify: do you suggest to get "less off-topic" Qs by relaxing our off-topic standards?
Answer: As with all SE sites, we get fewer closed questions when users read the help section and understand site requirements before posting. In particular, the section entitled What topics can I ask about here should be read and understood.
The community does close questions fairly regularly. Since a single moderator vote will close a question while it takes 5 non-moderator votes, it's natural that many questions end up being closed by mods. I actually let many questions I think probably need to be closed to remain open so I can see what the community thinks. The exception to that is questions requesting medical advice. I close those instantly because the community has long agreed that such questions shouldn't be allowed (and shouldn't be answered either).
We already have a disclaimer. Nonetheless, allowing medical advice questions isn't going to happen. That's an old debate that goes back to the founding of the site and it has always been widely agreed upon by the community. Please read this meta thread. If you want to get medical advice from anonymous strangers with unknown or non-existent qualifications, there is no shortage of sites on the internet where that can be found, but the community here has long agreed this isn't going to be one of them.
I'm not quite sure why you added the link in your question. It's from the very early days of the site when the scope was less rigorous, but even then it clearly stated that medical advice questions were off topic.
Comment: @LangLangC Do you mean https://medicalsciences.stackexchange.com/review/close/stats ? That only shows close votes cast from the review queue, if you access a post from the main feed it does not accrue there. I know my stats show way fewer than the number of close votes I've actually given, because there are few enough questions here that it's easy to see them all as they come in if you are active every day or two.
Comment: Interestingly, your name doesn't even show up in the CV-stats? How's that possible? Other mods appear in that list.
Comment: If you look at my profile under the votes tab, I've cast over 3500 votes, 1844 of which were close votes. I rarely look at the review queue because I see the new questions virtually every day, so I'm unlikely to miss much.
Comment: No. If *I* look at your profile the votes-tab doesn't expand. Perhaps that's different for diamonds, but ordinary users see that you cast 2418 votes (up&down). That's it. No info on reviews or CVs.
Comment: @LangLangC (shrug) I'm not really sure how that tabulation works. I've never paid much attention to it to be honest.
Answer: Closing a question is not a bad thing if it is a bad question.
Closing a question is not permanent: closed questions can be reopened if they are improved.
Closing a question is to prevent answers before a question is improved.
Not closing a question requesting personal medical advice is dangerous, because that allows it to be answered, and may require the moderators to then delete answers that are inappropriately providing medical advice. Even worse, those answers are most likely to come from people who are not regulars on this stack (because they do not understand the prohibition against personal advice) which makes them even more likely to be dangerous.
Some relevant posts about close-voting on the main meta that discuss the purpose of closing questions and recommend immediate rather than patient use of the close vote:
How soon should I "vote to close"?
What is a "closed", "on hold", or "duplicate" question?
What is the purpose of closing a question?
Comment: "Closing a question is to prevent answers." Maybe self-evident, but is that written out and reasoned/explained somewhere?
Comment: @LangLangC https://meta.stackexchange.com/questions/98022/how-soon-should-i-vote-to-close https://meta.stackexchange.com/questions/213382/what-is-the-purpose-of-closing-a-question https://meta.stackexchange.com/questions/10582/what-is-a-closed-on-hold-or-duplicate-question
Comment: What OP wants remains a bit elusive to me right now. But I think the comment links might be better placed within the answer. –– What I currently ponder about is the effect that closing has if there already are answers (here this mostly effects old Qs bumped to the frontpage…)
Comment: I do on occasion close old questions for being medical advice if I happen to come across them, and I've also deleted some answers to them. But with old questions I usually only do that when they're fairly egregious or they've attracted poor/incorrect answers. Otherwise I'm more likely to protect them.
Comment: Clarified slightly also, because what I meant to refer to is the *quick* closing of questions, since this is what the OP was mostly referring to (in advocating for more close patience). Preventing answers to questions that need to be improved before they are acceptable is the main purpose of closing questions quickly rather than being patient.
Comment: @LangLangC You're right; done. I agree that for old questions the purpose is a bit different, but not entirely. Old questions and their answers have to be viewed a little differently since standards change, but I'd argue that it's still good to prevent new answers to old questions that don't fit the modern standards of the site. Closing has a second, long-term purpose of eventually leading to deletion, which I feel applies equally to new and old questions.
Comment: But an upvoted answer prevents deletion? And if an answer exists for an OTQ then it not only stays free from roomba-action but also free from competing (perhaps needed/better) answers?
Comment: @LangLangC I did not realize closed questions with an answer are not roomba'd eventually. Might be worth a separate meta to discuss what we should do with such questions here. It does seem that closed questions can be voted for deletion after some time has passed, though: https://meta.stackexchange.com/questions/5221/how-does-deleting-work-what-can-cause-a-post-to-be-deleted-and-what-does-that

How can we get less off topic question?


Setting up Anaconda 2 with ArcPy

Question: I want to run Jupyter Lab (Notebook) with ArcPy. I have ArcMap 10.7 with Python Version 2.7.15, Numpy=1.9.3, matplotlib=1.5.2.
I followed the instructions from the following link:
https://gisday.wordpress.com/2016/07/18/setting-up-anaconda-pysal-with-arcgis-python-environment/
and I have downloaded Anaconda 2 32-bit.
I can only run the Anaconda Powershell Prompt as an Administrator, but this option does not exist for the Anaconda prompt (I do not know whether this makes a difference...).
The problem I have occurs when running the command:

conda create -n arc107 python=2.7.15 numpy=1.9.3 matplotlib=1.5.2
  pyparsing xlrd xlwt pandas scipy ipython ipython-notebook
  ipython-qtconsole

I get the following error:
PackagesNotFoundError: The following packages are not available from current channels:
then it lists: matplotlib=1.5.2, ipython-notebook, ipython-qtconsole
I deinstalled Anaconda and reinstalled it again, but it does not work. I also did not get the thing running with Anaconda 3, 32-bit.
Answer: We have some clues here that may help you
Using Anaconda modules from the ESRI python environment
(So sad I am retired and now "unknown user")

Setting up Anaconda 2 with ArcPy


Copyright Lyrics to Songs

Question: I have written lyrics to which I would like to have music put to. What's the safest and most advantageous way to copyright the lyrics?
Answer: If you are in the US, your lyrics were copyrighted the moment you put them on paper.  (I can't speak for other countries.)
If you want to register your copyright in the US to make it easier to protect your rights, go to https://www.copyright.gov/ .  They have instructions on doing it there.
Answer: To add to what Adam Stated, it is also important to have the lyrics dated.  One of the first things we were taught when I was in engineering was to date all of our design plans in our notebook so that if someone where to bring us to court over copyright issues, you have at least more evidence in your favor that this was your original work and done before them.  It would also be better to hand write because a date can be easily changed without notice on a word doc.  On a piece of paper, there would be an erase mark or white out used in the date section which would raise speculation to the authenticity of the changed date (and with removal of the white out you can see the original date as it would leave an ink trace or lead trace even if it was erased).  Not that this link provides how to copyright, but it may be an interesting read about what happens when a song goes to trial for copyright issues.
Comment: This is why I was taught to use a non-spiral notebook, where you can number the pages and see if pages have been torn out. Unless you are leaving pages empty to backfill with some new lyrics (poems/drawings/whatever) it is pretty clear what order things were done in. Date everything, non-spiral notebook, that's my suggestion.
Comment: The only problem with a hand-written date is that someone can back-date the document as long in the past as they desire, and claim that the work in question was written years ago. This method does not prove or secure anything.

Copyright Lyrics to Songs


Are these trim or bleed lines? And How can I add them to a design?

Question: So basically, I've made a set of playing cards for a client and he said the printer wants to add 3mm bleed lines to the cards like  .. However these don't look like bleed lines to me?(the pink line) Aren't bleeds on the outside of the image not inner? I'm guessing they're trim lines.. I've been looking for tutorials everywhere on how to add them with no luck.
Comment: You should ask the print shop what the marking means but you're right, it probably is a trim line.
Answer: This is a cut line, most likely for the print to get die-cut. You can create this with a stroke and then setting it to a spot colour called something like 'cutter'. The printer can then send this artwork to get a die made at the correct spec.
Answer: Your file should look more or less like this.

As @James C answered, it is a cut line. It should be on a separated layer. It does not matter the color of it.
But you also can send it on a different Pdf. I use the same name as the CMYK file and add My-PlayingCards-Cutline.pdf.
But your project should have trim lines. These will not be used to actually trim the paper, but to align the cut later on.

The "bleed lines" are the ones you use to define how much bleed you will add to your project, but they are only a reference for you. The printer does not need them. The trim lines are enough.
On this images I posted you can imply them.


English "whole" and Greek "holos"

Question: Is the English adjective "whole" genealogically related in any way to the adjective "holos", which means "whole" in Koine (and possibly other varieties of Greek; I'm not sure), and has a similar pronunciation? If so, how are they related?
Comment: @z7sg, Why did you delete your post? I couldn't even upvote it!
Comment: @Third Idiot I leapt to the completely opposite and wrong conclusion!
Comment: Ah, well, you've corrected it, and its equally great -- in the opposite direction.
Answer: They sound very similar but are actually unrelated.
The Proto-Indo-European root of whole is *koilas.  From etymonline:
whole (adj.)

O.E. hal "entire, unhurt, healthy," from P.Gmc. *khailaz "undamaged" (cf. O.S. hel, O.N. heill, O.Fris. hal, M.Du. hiel, Du. heel, O.H.G., Ger. heil "salvation, welfare"), from PIE *koilas (cf. O.S.C. celu "whole, complete;" see health). The spelling with wh- developed early 15c.

Greek holos is related to English safe and several other words, including Catholic.
safe (adj.)

late 13c., "uninjured, unharmed," from O.Fr. sauf, from L. salvus "uninjured, healthy, safe," related to salus "good health," saluber "healthful," all from PIE *solwos from base *sol- "whole" (cf. L. solidus "solid," Skt. sarvah "uninjured, intact, whole," Avestan haurva- "uninjured, intact," O.Pers. haruva-, Gk. holos "whole"). Meaning "not exposed to danger" is attested from late 14c.; of actions, etc., "free from risk," first recorded 1580s. Safe-conduct (late 13c.) is from O.Fr. sauf-conduit (13c.).
Comment: +1 for the great answer. I wish I could also give you another +1 for the link to etymonline.com !
Comment: Um, not great I messed um completely.  Fixed now. :) Etymonline is the greatest...
Comment: I'd like to link my answer in this question, since it's related: [Meaning of "hail to the king"](http://english.stackexchange.com/questions/26982/meaning-of-hail-to-the-king/26985#26985).

English "whole" and Greek "holos"


How to create tappered thread in OpenSCAD?

Question: Is there any simple way of creating tappered thread in OpenSCAD? I need something like 10 mm in diameter at the end, 9 at the top and the height of 10 mm.
Comment: Could you elaborate on what approach you took, what you've already tried and why it failed? That might help possible asnswers to know where to start explaining.
Comment: There's plenty of libraries for OpenSCAD for creating threads. Just couldn't find any for tapered thread (i.e. pipe thread).
Answer: If your math and OpenSCAD skills are superior to mine, you may be able to make use of the OpenSCAD Metric Nut, Bolt & Threads Library located here:
OpenSCAD Metric Nut, Bolt & Threads Library
It uses various means to generate polygons about a radius and includes the formulae for partial revolutions. It is presumed in the design that the center of rotation for the generated polygons is constant. I looked over the code for outside thread and could easily determine the radius references.
With proper coding, you could generate a variable radius based on the height of the cylinder at a specific point and achieve the tapered effect you require.
I expect that you'd have to reduce your desired radius by a fraction, say 0.05 mm in order to embed the thread forming polygons within your tapered cylinder.
If you aren't a strong coder, disregard this answer.
Comment: So, no ready solution. Thanks for the comment. Will play with the library.
Answer: I have contacted Dan Kirshner (the author of openscad threads library) and he has updated the library. Now it supports tapered threads. Thanks, Dan!
Thread-drawing modules for OpenSCAD


BLE client server communication

Question: This is probably asked several times before, but my websearch skills can't really direct me to an adequate answer.
I have a thingy, that takes commands from a mobile application with a request-response protocol.
Also from time to time it sends notifications to the mobile app.
Until now, I used RFCOMM, but iOS doesn't really seem to support it. So I had to switch the underlying implementation. So far I found that the GATT profile might be what I need, but I'm not sure if it is the right direction. Or if it is achievable at all.
So what profile/setup should I dig into to implement this communication pattern?
Comment: please provide a link to the answers you found ... state the reason why the answer was not adequate
Answer: Since you are focusing on iOS in your question, I can recommend looking at

Apple's Bluetooth Developer Web Site.
Take a closer look at Bluetooth profiles that iOS and iPadOS support
And Chapter 15 of Accessory Design Guidelines

From personal experience, I would recommend using an Espressif ESP32 as a development standin for your thingy, as you can learn a great deal about Bluetooth development.  Notably, developing with BLE for iOS interactivity is more flexible than classic Bluetooth, and, if I may share an experienced opinion, Espressif has many examples with good documentation.
Comment: Thanks a lot. I've already checked the first two of your suggested literature, but the third one is new to me, I hope it will help, I'll have some time in the weekend to try some stuff. And btw, we'll switch to esp32 from our current raspberry based thingy :)
Comment: Cool.  I appreciate the feedback.  If you want to say I answered your question, I'd appreciate it.
Comment: unfortunately I still don't know if a custom profile over GATT is the way to go, or something else. Maybe I missed something (and I also admit I haven't checked the Accessory Design Guidelines in depth yet).
Answer: The exact setup depends on what you want or need to do, but the basic scheme is that:

you define a service with your own custom UUID
you define a characteristic on that service (or several), with read/write/notify flags depending on your needs
you start advertising

This is a scheme that will work with all BLE devices with an appropriate app running on them.
Sometimes you can just advertise and not bother with the service and characteristics. Sometimes you use pre-defined services and characteristics. And there are probably other combinations.

BLE client server communication


Is it important for postdocs to have a "postdoc mentor"?

Question: I'm in pure math. It seems like "postdoc mentors" are common nowadays in U.S. universities. The idea is that you do a postdoc under the supervision of someone whom you can write papers with. My impression though is that a lot of mathematicians didn't have postdoc mentors. Would it be a bad sign to not have a postdoc mentor? Would it be negative for productivity and developing a publication record and recommendation letters?
Comment: Note that in many cases funding agencies are now attaching mentoring requirements to the grants that pay for postdocs.  For example, the NSF requires that proposals include a "Postdoctoral Researcher Mentoring Plan."
Answer: It is true that, long ago, when "post docs" were not as numerous as now, there was no idea at all that you'd have a mentor. Rather, even though you'd be a new PhD, you were "an expert". And, also, long ago, joint papers were not so common.
That culture has not disappeared, unfortunately. Yes, in my opinion, sensible people of_course think in terms of mentoring... But this viewpoint is not universal.
Joint papers are a yet-subtler thing: good to have a collaboration, but these days, still, a joint paper with a senior person (in math) may be less of a CV-boost than a solo paper. As an "old person" myself, I try to give useful advice to junior people without trying to get my name on their papers.
EDIT: to answer more of the question, ... letters of recommendation for your next job do need to come from people who have engaged with you. This doesn't have to be a "mentor", though obviously a person who takes that role should be in a good position to write a helpful letter. At least in the old days ^tm, a postdoc's goal was to make a good impression on at least a few senior people in the dept, for letters. Seminar talks were the usual way.
I think the letter-of-recommendation concern is more substantial than "productivity", in saner parts of math. :)
Comment: Does it make sense to have a postdoc mentor who is unfamiliar with your work and doesn't work closely in your area/ with whom there is little chance of collaboration?
Comment: @Mehta I'm not an academic, but I've found reasonable value in mentors who are decidedly not people who I'd go to for "technical" problems. The separation helps them focus on advising you in career/interpersonal/long-term questions that I think become muddy for people who are "too close" to your situation.
Answer: I think there is some confusion about what a "postdoc mentor" is actually supposed to do.
But, to start with, let me just say that I don't think anyone can be successful in an academic career without a mentor. A mentor is supposed to help a mentee go through their career by providing advice on the many things a newly graduated PhD simply cannot know: How to deal with reviewing papers, how applications are read by hiring committees, how to become a mentor to students themselves, how to deal with authorship disputes, how to write grant applications, and many other things. That is what a mentor is supposed to do, and because so many things in academia are sub-discipline-specific, it only makes sense that such a mentor comes from the same sub-discipline.
That often leads to joint papers, but that is a side effect and not the main reason to have a mentor for postdocs. The main reason is simply to ensure that the people we hire into postdoc positions don't fall through the cracks but are successful in their careers!
Comment: @PeterShor I don't actually know whether Einstein had a mentor, but (i) I think it's not useful to compare anyone to Einstein, nor (ii) is an example from 115 years ago useful for today.
Comment: Did Einstein have a mentor in the Swiss patent office? It's incredibly useful to have a mentor, but not everybody actually needs one. You should have a mentor, and you should go to them if you have problems with or questions about anything, but many postdocs don't interact with theirs that much (some do!).
Comment: That's in such contrast with the advice I had been given: don't worry about postdoc mentors. Just focus on publishing.
Comment: "how to write grant applications" <-- this is probably the biggest help, along with advice around looking at applying for a more permanent position.
Answer: This depends on the individual and the department that you join, in math, anyway. If there are a few (or a lot) faculty in your specialty and they hold periodic seminars to discuss one another's work, then a mentor probably isn't needed.
But if you still feel a bit shaky in your research ability, then a mentor might help give you a boost. I don't think it would be a bad sign to not have a mentor (or to have one), but you want to make enough faculty contacts and earn the respect of your colleagues so that you get a boost for your next steps.
Of course, there are a lot more joint papers these days than there were in the past. You want the opportunity for collaboration, however you can arrange it.


Write an android application

Question: What is the best way for me to go about writing a new Bitcoin wallet application? Is piggybacking off API's like blockchain.info or coinbase a good tactic? Would the application then be OK to distribute (are said services open source?). Are there alternatives? Can I run my own Bitcoin server and write my own API to go with it instead? What would the steps be involved in that? Would your average punter trust a (legitimate!) wallet that I create or would they feel safer using an app that is built upon a renowned service?
Answer: You can fork Bitcoin Wallet, an SPV implementation of the Bitcoin protocol:
https://github.com/schildbach/bitcoin-wallet
It's GPL licensed.
Answer: A hosted wallet, which will involve a wallet running on a server that has all the wallets is a bad idea in general. People will not feel safe as they have to trust a third party with their coins and you'd have to implement a whole backend system.
Piggying off Coinbase or a Blockchain is a bad idea, the services themselves may send you a Cease and Desist as you may send a lot of requests and end up swamping their servers, also you'd be copying the functionality of their apps.
You can write a full node, but a mobile phone may not have enough ram and storage to efficiently and quickly do that.
The only nice, viable and realistic approach is creating an SPV client like Electrum for the mobile platform. SPV clients aren't full nodes, they put a certain amount of trust on third parties, but the amount of trust is incredibly small and always double checked. Their main method of work is just requesting data from servers, etc. You can use the Electrum protocol instead of rolling your own as well, you won't need to host servers as well doing that as the community runs Electrum nodes themselves.
Comment: "You can write a full node, but a mobile phone may not have enough ram and storage to efficiently and quickly do that." Can I not write a full node on my computer and query that with mobile device? It has been mentioned that the JSON API is an option?
Comment: @JoeAustin That will be the same as the 1st or 4th option depending on what do you want to delegate to your main computer. The problem is that having the full blockchain is not an option for mobile so you need some way to trust someone and delegate the process of getting just the transactions you need while being sure that the they are real.
Comment: My phone is more powerful than my PC...
Answer: Take a look at this:
www.boetchain.com or https://play.google.com/store/apps/details?id=com.boetchain.bitcoinnode
It still very early, so for now, only maintaining connections with peers are available - but it is a start.
Disclaimer: I am a dev on the project

Write an android application


ISS Speed required for true geosynchronous orbit

Question: How fast would ISS need to travel at the current altitude (LEO 250mi) to be geosynchronous with say 36 degree N lattitude?
Comment: The question is premised on a misconception. Without continuous thrust (which is entirely impractical) there's only one possible speed for circular orbit at a given altitude.
Comment: And also on the misconception that a geostationary orbit is possible about anything but the equator.
Comment: @DavidHammen OP did say "-synchronous" rather than "-stationary" at least, though it still doesn't work with any of the the other constraints.
Comment: @uhoh - OP also say "with say 36 degrees N latitude", which doesn't make sense.
Comment: @DavidHammen I know, the consistency has a damping range of about +/- 3 words.
Answer: If the ISS were to decrease speed to try to be geosynchronous it would start to fall in it's orbit.  The only way to keep it in its current orbit and be moving slow enough to be geosynchronous you would have to apply thrust backward against its current direction and constantly apply thrust up away from the Earth to counter the ISS from falling in altitude.
The speed at geosynchronous orbit is 3 km/sec, the speed of the rotation of the Earth at the equator is 0.46 km/sec, geosynchronous orbit is at 35,786 km and the ISS is at 400 km, the current speed of the ISS is 7.66 km/sec so the speed the ISS would need to travel to match the ground would only be slightly greater than 0.46 km/sec.
Comment: The edits completely changed the meaning of the answer...
Answer: Actually, the ISS would have to slow down by an order of magnitude in order to take 24 hours to orbit the Earth at its current altitude where it completes an orbit every 1.5 hours - 16 times a day - with an orbital velocity of 4.76 mi/sec.  Thus, to keep it "geosynchronous" at its current altitude, you would have to slow it to 1071 mph (4.76 mi/sec / 16 * 3600 - current speed divided by orbits per day times seconds per hour).  At that speed, it wouldn't be in orbit any more, so you'd have to be expending a LOT of energy as upward thrust to keep it from falling straight down from the sky like a brick.
Comment: @BrooksNelson Welcome to life ;)
Comment: I swear I caught myself and corrected mine before I read yours, I felt dumb enough getting it reversed, the ground speed threw me off.
Answer: AFAIK it's not possible for it to be Geosynchronous at it's current altitude. Geosynchronous orbits are only possible at a much higher altitude. At any given altitude there is only one speed which produces a stable orbit. Any faster and you'll spiral outward and any slower and you'll spiral inward. There's only one altitude where the correct orbital speed results in a 24 hour orbital period which is what produces a Geosynchronous orbit.
Comment: Sans atmospheric drag, orbits do not spiral in or out. They instead form conical sections (circles, ellipses, parabolas, and hyperbolas).


Are our mods taking part in the walkout?

Question: Mods,
Just checking if y'all are doing the walkout thing over the AI policy.  Not saying you should go either way, just wanting to know if you are.
Answer: I have signed the letter and marked myself as absent.
However, I have not handed back any of my mod diamonds. I would emphasize that signing the letter to the company is really just a walkout with the hopes that we can find a resolution. There have been moderators that simply stepped down instead.
The main reason I participate are the users here and on the other sites that I am a moderator on. When I got elected, I gave a commitment to this community to support and protect all of you and the site. If the company puts me in an position where I am forced to do one thing, when the public policy says another, when I am supposed to accept that mutual agreements, established after the 2019/2020 situation that almost tore up the network, are ignored - my conscience tells me to take a break. I feel my personal integrity is on the line here.
I will

still visit the site very regularly
not step down at the moment (but that's not entirely under my control)
still use my permissions to protect the individual users e.g. in cases of severe CoC violations, if no other moderator is available.

If you have any questions, I will be happy to talk with you in the Frying Pan.
Answer: I'm not very active at all, but I'll be sticking around for the time being to make sure that flags get handled (eventually, anyways).
This doesn't mean I'm happy about AI policies etc etc, just that I'm not deciding to go inactive over it. I'm also happy to leave the other mods free to do as they like.
For what it's worth, I'm strongly against many proposed uses of AI, including the use of AI to write answers here. I do not pretend to have done sufficient due diligence to know how best to implement policies, though. There are absolutely legitimate concerns about collateral damage and bias with any policy and means of enforcement I am currently aware of. My main hope is that the strike/walkout is part of a story that eventually leads to a better situation.
Answer: I am here, and moderating.
The AI question is still mostly theoretical for our site. We have deleted maybe a dozen posts on suspicion of being generated, and I don't think we've ever suspended anybody for AI-related reasons. So, unlike other moderators on the network, we aren't much affected by the policy itself.
As for the relationship between the company and the moderators, that's a difficult topic -  and from my point of view, the more important one. There are many facets of it which I'm not happy about. Still, currently, it doesn't feel like going on strike now would be the right decision for me personally. If this changes in the future, I'll let you know.
Answer: As said in the comments, I've been back in school full time. My long-term, barely active status puts me in the lovely position of not needing to make a decision about this today. I am not up to speed, so I'm not in a position to sign the letter or mark myself inactive.
If the striking moderators are fired, that would most likely cause me to give up my diamond. I hope it doesn't come to that.
From what I understand about the strike, I stand in solidarity with strikers.


What is chaotic behavior and how it is achieved in non-linear regression and artificial networks?

Question: I'm finding it hard to understand the relationship between chaotic behavior, the human brain, and artificial networks.  There are a number of explanations on the web, but it would be very helpful if I get a very simple explanation or any references providing such simplifications.
Comment: Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/83416/discussion-on-question-by-novice-venkat-what-is-the-chaotic-behavior-and-how-it).
Answer: It looks like you have some common misconceptions about AI and neural networks.
First, AI programs generally do not try to imitate the human behaviour of a human brain. Instead, they try to imitate some higher-level behaviour. For example, they might imitate the reasoning process that you go through when you make a plan. In this context, the building-blocks (silicon or flesh) don't matter too much.
Second, artificial neural networks are also (mostly) not intended to imitate the human brain. Although they are inspired by the arrangement of neurons in a human brain, the networks used in most ANN systems have very little to do with real brains. The main similarity is that both systems have a lot of simple little computational units connected in patterns such that signals passed from one to another lead to interesting computations. However, real neurons produce lots of different kinds of signals, are connected in arbitrary ways, and randomization and transmission times play a significant role. Artificial neurons generally are deterministic, produce only one kind of signal (or sometimes a couple different kinds), are connected in extremely regular ways, and usually simulate instantaneous transmissions between neurons.
Answer: Regression for models more complex than $y = a x + b$ is a convergence strategy. Surface fitting algorithms, such as Levenberg–Marquardt, are often successful at achieving regression using a damped version of least squares as an optimization criterion. The marriage of regression and the multilayer perceptron, an early model artificial network, led to the use of a back propagation strategy to distribute corrective signals that drive regression.
Back propagation using gradient descent is now used in artificial networks with a variety of cell and connection designs, such as LSTM and CNN networks as a convergence strategy. Both surface fitting and artificial network convergence share the method of successive approximation. With each successive application of some test, the result is used to attempt to improve the next iteration. Proofs have developed around convergence for many algorithms. Actual successive approximation runs have five possible outcomes.

Convergence within the time allotted and within the accuracy required
Convergence within the time allotted but not within the accuracy required
Convergence appears that it would have occurred but time allotted was exceeded
Oscillation appeared by the end of time allotted
Chaos appeared by the end of time allotted

The following illustration from Chaos Theory Tamed (Garnett P. Williams, 1997, p 164) modified slightly for easy viewing can explain how chaos arises when the learning rate or some other factor is set too aggressively.  The graphs are of the behavior of the logistic equation $x_{i+1} = k x_i (1 - x)$ which plots as an inverted parabola in phase space.  The one dimensional maps on the right of each of five cases show the relationship between adjacent values in the time series on the left of each of the five.  Although the logistic equation is quite simple compared to regression algorithms and artificial nets, the principles involved are the same.

The right hand cases, with $k = 3.4$ and $k = 3.75$ correspond to the last two possible outcomes in the list above, oscillation and chaos respectively.
Care in Drawing Parallels
Care should be taken in drawing parallels between distinct things.

Surface fitting algorithms, like Levenberg–Marquardt
Algorithms that realize back propagation with gradient descent
Logical inference AI, such as production systems and fuzzy logic
Real time learning, such as Q-learning
Devotion of the human brain to a problem

Regression and artificial networks can be compared meaningfully because the math for each is fully defined and easy for those with the mathematical skill to analyze them for the comparison.
Comparing known mathematical systems with unknown biological ones is interesting, but to a large degree, grossly premature.  The perceptron, on which MLPs (multilayer perceptrons) and their deep learning derivatives are based, are simplified and flattened models of what was once thought to be how neurons in the brain work.  By flattened is meant that they are placed in the time domain where they are convenient for looping in software and do not take into consideration these complexities.

Neuron behavior is sensitive to the timing of incoming signals — Incoming signals may overlap but not precisely align in time.
Neuron behavior is sensitive to the history of incoming signals (because of cell body and axon thermodynamics, synaptic chemistry, and other neuro-chemical and structural functions not yet understood)
Neuron structure changes in terms of its connectivity
New neurons appear
Neurons may die due to cell apoptosis

In summary, multilayer perceptrons are not a model of neural networks in the human brain.  They are merely roughly inspired by obsolete knowledge of them.
Chaos in the Human Brain
Whether there is chaotic behavior in the brain is known.  It has been observed in real time.  How coupled it is with human intelligence is a matter of conjecture, but it is already clear that it may appear to contribute to function in some cases and contribute to dysfunction in others.  This is also true in artificial systems.

When used to deliberately interfere with a stable condition that may not be the optimum state of stability to find a better one, chaos may be a source of noise that benefits learning.  This is related to the difference between local minima and global minimum.  The good is sometimes the enemy of the best.  Improved learning speed has been documented for artificial network algorithms with deliberate injection of pseudo-random noise into the back propagation.
When appearing not to deliberately inject noise in a portion of the system that can benefit from the noise but out of a basic instability in the system, the chaos can be detrimental to overall system function.  Chaotic behavior in the human brain is a likely cause for various disorders.  There is much supporting data but not yet a proof.

In summary, chaos in a system is neither productive nor counterproductive in every case.  It depends on where it is in the system (in detail) and what the system design is expected to perform.


Finite element method, question about the solver

Question: So I am working with COMSOL Multiphysics and I have a very complex simulation. I have a boundary condition that evaluates to 1 if the value of a variable is 20 and 0 otherwise. I could implement my code just fine in COMSOL. However, I am facing a problem (see a draft that I made below please). Basically, for the first iterations whenever the variable reaches 20 it turns the boundary condition to 0, but as I have more iterations the limit slightly increases (thus reaching values above 20). After some time the limit comes back to 20 and the same problem as before occurs again. Any thoughts?
I have already tried to increase the solver tolerance but the some problem occurs
Thanks in advance
Comment: I can't see any draft
Answer: In a nonlinear problem where you change some parameter, there is no guarantee that solutions for all values of the parameter will be stable. It is quite likely that for "small" values of the applied "load" (whatever that might be - not necessarily a physical force) there are stable solutions with your variable $x < 20$ and the boundary condition = 0, and for "large" loads there are stable solutions with $x > 20$ and the boundary condition = 1, but there is an intermediate range of loads with no stable solutions.
To figure out what is going on, I suggest you make three simulations with different conditions:

fix the "0 or 1" boundary condition to 0 for all values of $x$.
fix it to 1 for all values of $x$.
Change the boundary conditions in the model so that you fix $x = 20$ and let the "0 or 1" quantity vary.

You will probably find there are some situations where there are stable solutions with $x = 20$ but your "0 or 1" variable has values between 0 and 1. Most likely, there are no stable solutions of your original model in that region. For example you may find there is a stable solution with $x > 20$ and the boundary value = 0, and another stable solution with $x < 20$ and the boundary value = 1.
Since we don't know anything about your model, it's impossible to say whether the fix is to change the model to get rid of the instabilities, or use a more sophisticated "path following" solution method (for example the Riks algorithm, or the "arc length" method - two names for pretty much the same thing) which can find the unstable solutions without failing numerically.
Answer: Instead of having a hard 0/1 binary boundary condition try having a smoth and continuous one, try using the tanh() function for instance


Displaying Syntax highlighting and indentions. Specifically for Java, C#?

Question: I've seen other options for some languages, like PHP, HTML and CSS, but nothing for Java and C#.  Also it needs to be current, for Joomla 3.6.
Answer: CedHighlightjs (free) and JExtBOX Code Display (paid) seem to support highlighting of Java and C.
There may be others.
See the answer at https://joomla.stackexchange.com/a/18817/120 for some other possible options.


If cleared to cross a runway and expedite but are slow to get away, are you obliged to stop?

Question: Suppose ATC says "Big Jet, cleared to cross runway 36, expedite" and you acknowledge "Cross runway 36, wilco, Big Jet". As you start to move forward  you immediately realize that you are unable to go as quickly as you expected (perhaps there's an incline or a gust of wind -- I'm not sure what's plausible). At the point you realize this, you can still hold short the runway. Are you obliged to stop and reply 'unable' or is that ill-advised?
Comment: Just to nitpick...you are never "cleared" to cross a runway, since that is an instruction, not a clearance. So the phraseology used would be "Cross runway 36", not "Cleared to cross runway 36". There's a number of reasons for this, one of them being that hearing "cleared" and "runway 36" in the same sentence can result in some unlucky associations (pilot thinking he is cleared for takeoff etc.)
Answer: If you feel you cannot safely comply with any ATC instruction, you can simply reply "Unable" and the controllers will amend your clearance.  Remember: you are pilot in command; you are the final authority on the operation of that aircraft.
So the scenario might play out as:
ATC: "Bigjet, cleared to cross Runway 36.  Expedite crossing and report clear."
Bigjet: "Cross runway 36 and report clear, Bigjet."
Bigjet: "So-And-So Ground, Bigjet, Unable.  I've got a problem here."
ATC:  "Bigjet roger hold short of Runway 36 for arriving traffic.  Will you be able to taxi after traffic clears the active?"
Answer: Per Part 91, You are cleared to cross until/unless another clearance is obtained. https://www.law.cornell.edu/cfr/text/14/91.123
Answer: If you don't think you can safely comply with an instruction, don't comply with it. If you can remain short of the "hold short" line, stop there and inform ATC. If you have already crossed the line call tower and say "crossed hold short, unable to expedite" or something to that effect so they can have landing traffic go around.
Its been said quite a few times, the pilot is in control of the plane, not ATC. You can agree to something and then later realize you can't comply, that is fine, as long as you communicate with ATC.


What kind of Mac to buy?

Question: I've just started doing live sound at my local theater company and we might get a new Mac as opposed to using our antiquated Acer notebook any more.  For the upcoming show, we need to use QLab to play the sound effects and music.  I've used it some on my own MacBook Pro which has a 2.4 GHz Intel Cor 2 Duo and QLab runs really well.  The guy in charge of the company is thinking about buying a Mac and I'm just wondering if maybe we could save some money by getting a MacBook Air instead of the Pro.  Would QLab still work well on the MacBook Air?  Any advice would be great, thanks so much.
Answer: Mac Minis are great for Qlab audio, and are also cheaper than a macbook pro or a macbook air.  We have several mac minis at my school theatre (Ithaca College) for just this purpose.  If you're looking to do video though, make sure you use something with a discreet graphics card (ie. NOT a macbook air or 13" macbook pro).
Answer: I would personally not want a macbook air for any creative high bandwidth work. They're cool for a light thing to carry around, but they're underpowered for things like audio and video. Macbook pro would be my answer.
Answer: I agree with Dave, the Air would not be a good choice.
If you want to stay inexpensive, you should consider a Mac Mini.
Lots of theaters and major touring companies use them to run Qlab.

What kind of Mac to buy?


How can I scold my juniors without annoying them?

Question: I'm a senior at my school. A socially-awkward prefect, actually. Because of my shyness, I often ignore my juniors' misbehaviours, leading them to be spoilt. Also, they don't really care about what we seniors say because most of us are shy too. Our teachers are mad with the juniors because of their uncleanliness, rudeness and poor academic results, to name a few. So, how can I(we) scold them, just for the sake of the lesson, without annoying them?
Comment: I think strongly that there's a cultural aspect here. Please add information about your culture so that answers can be tailored to it.
Comment: Counter question: when you're a junior at that school, did the seniors scold you (or other juniors) because of their misbehavior? If yes, how did they do that? If no, why do you feel you have to scold them?
Comment: How come it's down to you to 'scold' them as you say? Why isn't it the responsibility of the teaching staff? If it is just larkspit, then I'd let it fly and if it is anything more serious, contact the teaching staff.
Comment: In particular, as someone in the U.S., I only sort of know what a "prefect" is from Harry Potter. What does that role mean at your school, and what its responsibilities and powers?
Answer: Instead of scolding them try to appreciate the people who behave polite/clean/score good marks. You can offer chocolates/cookies to them.This will make the other children to behave good. If you are studying in a school, then I am sure your juniors will be fond of chocolates/cookies. You can also ask their teaching staff to give bonus points to them for their neat and polite behavior. By this way, you can make your juniors behave good without scolding them.
Answer: A general look on such a situation. Scolding tells them you are not happy with them. If they don't care what others think, this will have no effect. Worse - it will have a negative effect as it decreases their respect for you.
What you need is something to make them not happy with themselves.
How? This depends on your role in this school. I don't understand this junior-senior relationship and why teachers are mad about them but you should correct this. So it may be the best first step to talk to your teachers about that.
Answer: A lot of this depends on the relationship you have with your juniors.  I'll approach this from the US perspective.
If your relationship is based solely on power, then there may not be a lot of respect there.  That makes it harder to make the scolding be effective.
I assume you can't assign discipline to the juniors (back in grade school, it was clapping out erasers.  You'd go outside and clap erasers to clean the chalk dust out.  It was dusty, dry, and not pleasant.  No one does that any more...)
I'd start by focusing on one thing.  Pick the most egregious one and deal with that.  Compare it to something relevant that really matters to them.  "Adults act more politely with others than you are right now.  I'm, quite frankly, disappointed in this."  "How often do you see [famous person that they idolize] in the news for acting like this?  Sure, they may do it once in a while, but not all the time - which is what I see here.  I just don't imagine this happening with him/her."  "You would talk to your mother that way?  Really?"  That may make the issue relatable to them.


Why don't the WHO zombies frantically bash their heads through glass like the earlier zombies?

Question: Early in the film, we see zombies headbutt car windows in order to reach their prey, but the ones inside the WHO facility do not seem to use that tactic. It is possible that some of the facilities glass is shatterproof, but the broken glass strewn about the place proves some of it is not. Furthermore, the female scientist zombie doesn't show the signs of head trauma one would expect if she had encountered glass she couldn't break through with her forehead. I may have missed the reason in the film, or maybe the book holds the answer to Why didn't the latter zombies frantically bash themselves to get the humans beyond the separating glass?
Comment: I think the answer is as simple as 'the disastrous third act was written by different screenwriters, and therefore bears very little similarity to the previous two-thirds of the film.' The book and the film are entirely different; they merely share a title and a few character names, much like how Will Smith's *I, Robot* has nothing in common with Asimov's anthology. You won't find an answer to film plot holes in there.
Answer: To the best of my knowledge, there is no in-universe explanation for this.  Like almost everything else about the movie, the scene in question does not occur in the book.
The production history of the movie World War Z was particularly torturous and difficult.  The third act (basically, everything from the plane crash through the end of the film) was rewritten a number of times, and the version we saw on our screens was wildly different from the original script.
As James Sheridan has already pointed out, the final version of the third act was written by a different group of people than the first two acts. This might have made it harder to achieve a sense of continuity and consistency throughout the entire movie.
I'm afraid that any other explanation would necessarily involve a significant amount of speculation:  Maybe the zombies in the opening scenes were whipped up into a frenzy by the large number of people around them, whereas the WHO zombies were isolated in an area devoid of any humans for several days, and were therefore relatively calm.  We just don't know, and it is difficult to imagine that we will ever find out.
Comment: I am looking for a quote, but I am pretty sure that it said that the zombies go dormant when humans are not around. If I have the time to find it I will probably write an answer out.


Are carousels effective on non-Ecommerce sites

Question: We run digital magazine websites. We use a carousel to feature content. Since we are interested in the user clicking through to view the content, and not converting them to anything (these are inhouse or association magazines, so no outside subscribers). Is this still a problem? Does the end user not see the carousel, or only sees the first image?
Comment: In my view, the other question covers all cases, including yours. Can't think of anyone jumping in saying "but in non-ecommerce carousels are effective". It's like banner blindness - matters little on which type of site it is.
Comment: @Benny That proposed duplicate asks specifically about conversion on e-commerce sites.  This one is about other type of sites and different engagement metrics.  It does not appear to be a duplicate.
Comment: @3nafish I have to agree with Izhaki here. UX doesn't change between scope. From a users perspective a carousel on an Intranet is no different than a carousel on a shopping site.
Comment: @Izhaki, I agree that the ineffectiveness of carousels is likely to be independent of site type, but just because this question is likely to receive similar answers doesn't mean it's a duplicate question.
Comment: @BennySkogberg, A shopping site and an intranet have different target users and different contexts, so there could conceivably be some difference in the way people act.  The key difference in this question is the way that "effectiveness" is operationalized.  The OP is asking about click-throughs (presumably as a metric of engagement) while the other question was concerned with conversions (presumably as a metric of monetization).
Answer: Have the options to flick through the carousel but not auto slide - maybe the most important click through or link to content on the first slide and let the user perform the click through's if they want to.
We have a similar scenario with our corporate site, we aren't selling anything per se, but want to improve the landing page aesthetically.
Good piece on this from NN group:
http://www.nngroup.com/articles/auto-forwarding/
Cheers,
Answer: According to research done by NN Group, and as per answers in similar existing questions , generally carousels are implemented in a manner that is not very user friendly. The problem with carousels (whether auto-scrolling or not) is that it's very easy for content to be missed by the user.
One of the few appropriate uses for carousels are when you are displaying multiple units of content that all belong to the same category, or in scenarios where missing a carousel item does not hamper the main message of the page. An example of this is when Apple shows off some sample apps in a carousel on their iPad page. The message is that the iPad has access to apps, and if you browse through the items in the carousel, you will see more of those sample apps. This implementation is usable because even if you miss the secondary carousel items (additional sample apps), you still understand the main message which is that the iPad has access to apps.
In contrast, if you're showcasing content on a homepage, putting the top three stories in a carousel is a bad idea because it's very easy for users to miss them. Why run the risk of having users miss out on them? For content sites, it's preferable to have correctly written headlines and thumbnails that are easy to scan at a glance.