Why liquidity implies tight spread and low slippage
Consider the case where a stock has low volume.  If the stock normally has a few hundred shares trade each minute and you want to buy 10,000 shares then chances are you'll move the market by driving up the price to find enough sellers so that you can get all those shares.  Similarly, if you sell way more than the typical volume, this can be an issue. Theoretically, it's a question of rate of return. If a desired or acceptable rate of return for market makers' capital is X, and X is determined by the product of margin & turnover then higher turnover means lower margin for a constant X.  Margin, in the case of trading, is the bid/ask spread, and turnover, in the case of trading, is volume. Empirically, it has been noted in the last markets still offering such wide-varying evidence, equity options: http://faculty.baruch.cuny.edu/lwu/890/mayhew_jf2002.pdf You have just answered your question in the last sentence of your question:  More volume just means more people are interested in the stock...i.e supply and demand are matched well. If the stock is illiquid there is more chance of the spread and slippage being larger. Even if the spread is small to start with, once a trade has been transacted, if no new buyers and sellers enter the market near the last transacted price, then you could get a large spread occurring between the bid and ask prices. Here is an example, MDG has a 50 day moving average volume of only 1200 share traded per day (obviously it does not trade every day).  As you can see there is already an 86% spread from the bid price. If a new bid price is entered to match and take out the offer price at $0.039, then this spread would instantly increase to 614% from the bid price.
If the stock has low liquidity, yes there could be times when there are no buyers or sellers at a specific price, so if you put a limit order to buy or sell at a price with no other corresponding sellers or buyers, then your order may take a while to get executed or it may not be executed at all. You can usually tell if a stock has low liquidity by the small size of the average daily volume, the lack of order depth and the large size of the gap between bids and offers. So if a stock for example has last sale price of $0.50, has a highest bid price of $0.40 and a lowest offer price of $0.60, and an average daily volume of 10000 share, it is likely to be very illiquid. So if you try to buy or sell at around the $0.50 mark it might take you a long time to buy or sell this stock at this price. In my experience thanks to algorithmic trading the variation of the spread and the range of trading straight after a major data release will be as random as possible, since we live in an age that if some pattern existed at these times HFT firms would take out any opportunity within nanoseconds. Remember that some firms write algorithms to predict other algorithms, and it is at times like those that this strategy would be most effective. With regards to my own trading experience I have seen orders fill almost €400 per contract outside of the quoted range, but this is only in the most volatile market conditions. Generally speaking, event investing around numbers like these are only for top wall street firms that can use co-location servers and get a ping time to the exchange of less than 5ms. Also, after a data release the market can surge/plummet in either direction, only to recover almost instantly and take out any stops that were in its path. So generally, I would say that slippage is extremely unpredictable in these cases( because it is an advantage to HFT firms to make it so ) and stop-loss orders will only provide limited protection. There is stop-limit orders( which allow you to specify a price limit that is acceptable ) on some markets and as far as I know InteractiveBrokers provide a guaranteed stop-loss fill( For a price of course ) that could be worth looking at, personally I dont use IB.  I hope this answer provides some helpful information, and generally speaking, super-short term investing is for algorithms. And the fact that if notes trade up high enough they tend to get taken out with lower yielding notes (pending the call schedule), so in theory there is more limited upside in high-yield, whereas equity could theoretically trade to infinity.  The positives for high-yield is that they generate monthly cash flow via coupon payments and have less down-side relative to equities. Naturally this lower down-side comes at the expense of lower up-side as well. You can't tell for sure.  If there was such a technique then everyone would use it and the price would instantly change to reflect the future price value. However, trade volume does say something.  If you have a lemonade stand and offer a large glass of ice cold lemonade for 1c on a hot summer day I'm pretty sure you'll have high trading volume.  If you offer it for $5000 the trading volume is going to be around zero. Since the supply of lemonade is presumably limited at some point dropping the price further isn't going to increase the number of transactions.   Trade volumes reflect to some degree the difference of valuations between buyers and sellers and the supply and demand.  It's another piece of information that you can try looking at and interpreting.  If you can be more successful at this than the majority of others on the market (not very likely) you may get a small edge.  I'm willing to bet that high frequency trading algorithms factor volume into their trading decisions among multiple other factors. I understand the question, I think. The tough thing is that trades over the next brief time are random, or appear so. So, just as when a stock is $10.00 bid / $10.05 ask, if you place an order below the ask, a tick down in price may get you a fill, or if the next trades are flat to higher, you might see the close at $10.50, and no fill as it never went down to your limit.  This process is no different for options than for stocks.  When I want to trade options, I make sure the strike has decent volume, and enter a market order. Edit - I reworded a bit to clarify. The Black–Scholes is a model, not a rigid equation. Say I discover an option that's underpriced, but it trades under right until it expires. It's not like there's a reversion to the mean that will occur. There are some very sophisticated traders who use these tools to trade in some very high volumes, for them, it may produce results. For the small trader you need to know why you want to buy a stock or its option and not worry about the last $0.25 of its price. While the Vanguard paper is good, it doesn't do a very good job of explaining precisely why each level of stocks or bonds was optimal. If you'd like to read a transparent and quantitative explanation of when and why a a glide path is optimal, I'd suggest the following paper:  https://www.betterment.com/resources/how-we-construct-portfolio-allocation-advice/ (Full disclosure - I'm the author).  The answer is that the optimal risk level for any given holding period depends upon a combination of:  Using these two factors, you construct a risk-averse decision model which chooses the risk level with the best expected average outcome, where it looks only at the median and lower percentile outcomes. This produces an average which is specifically robust to downside risk. The result will look something like this:   The exact results will depend on the expected risk and return of the portfolio, and the degree of risk aversion specified.  The result is specifically valid for the case where you liquidate all of the portfolio at a specific point in time. For retirement, the glide path needs to be extended to take into account the fact that the portfolio will be liquidated gradually over time, and dynamically take into account the longevity risk of the individual.  I can't say precisely why Vanguard's path is how it is. Slippage is tied to volatility, so when volatility increases the spread will also increase. There is no perfect formula to figure out slippage but from observations, it might make sense to look at the bar size in relation to previous bars to determine slippage (assuming fixed periods). This is because when there is a sudden spike in price, it's usually due to stop order triggering or a news event and those will increase the volatility dramatically in seconds. "High liquidity doesn't necessarily mean that ""everybody is getting rid of the stock"", since somebody is obviously buying whatever stock that is being sold. Also, as mentioned, low liquidity may mean that you would have trouble selling the stock in the future." I'd think that liquidity and speed are prioritized (even over retail brokers and in come cases over PoP) for institutional traders who by default have large positions.  When the going gets tough, these guys are out and the small guys - trading through average retail brokers - are the ones left holding the empty bag. a smaller spread indicates a flat yield curve, which means banks and investors are uncertain about future economic conditions (like the current environment).   When the spread widens and the curve becomes upward sloping (considered a normal yield curve), investors expect future growth and minimal inflation. Longer term rates increase as investors demand a higher yield in return for lending their money for a longer period of time. Increase demand for credit (industries expanding) also drive up longer term rates.   A negative spread indicates an inverted yield curve and investors believe the economy is overheating and interest rates will fall. Investors pull money out of the stock market and into long term bonds (raising the price, lowering the yield) while companies stop borrowing, reducing the demand for credit and lower the cost, or interest rate, on a loan.   Keep in mind central banks determine short term rates, so inverted curves are rare in the sense the market perceives uncertainty and rushes to safety (bonds) before the central bank reacts and lowers short term rates. In the stock market many participants enter orders that are not necessarily set at the current market price of the stock (i.e. they are not market orders, they are limit orders). They can be lower than the market price (if they want to buy) or they can be higher than the market price (if they want to sell). The set of orders at each point of time for a security is called the order book. The lowest selling price of the order book is the offer or ask, the higher buying price is the bid. The more liquid is a security, the more orders will be in the order book, and the narrower will be the bid-ask spread. The depth of the order book is the number of units that the order book can absorb in any direction (buy or sell). As an example: imagine I want to buy 100 units at the lowest offer, but the size of the lowest offer is only 50 units, and there is not any further order, that means the stock has little depth. "There are people (well, companies) who make money doing roughly what you describe, but not exactly. They're called ""market makers"". Their value for X% is somewhere on the scale of 1% (that is to say: a scale at which almost everything is ""volatile""), but they use leverage, shorting and hedging to complicate things to the point where it's nothing like a simple as making a 1% profit every time they trade. Their actions tend to reduce volatility and increase liquidity. The reason you can't do this is that you don't have enough capital to do what market makers do, and you don't receive any advantages that the exchange might offer to official market makers in return for them contracting to always make both buy bids and sell offers (at different prices, hence the ""bid-offer spread""). They have to be able to cover large short-term losses on individual stocks, but when the stock doesn't move too much they do make profits from the spread. The reason you can't just buy a lot of volatile stocks ""assuming I don't make too many poor choices"", is that the reason the stocks are volatile is that nobody knows which ones are the good choices and which ones are the poor choices. So if you buy volatile stocks then you will buy a bunch of losers, so what's your strategy for ensuring there aren't ""too many""? Supposing that you're going to hold 10 stocks, with 10% of your money in each, what do you do the first time all 10 of them fall the day after you bought them? Or maybe not all 10, but suppose 75% of your holdings give no impression that they're going to hit your target any time soon. Do you just sit tight and stop trading until one of them hits your X% target (in which case you start to look a little bit more like a long-term investor after all), or are you tempted to change your strategy as the months and years roll by? If you will eventually sell things at a loss to make cash available for new trades, then you cannot assess your strategy ""as if"" you always make an X% gain, since that isn't true. If you don't ever sell at a loss, then you'll inevitably sometimes have no cash to trade with through picking losers. The big practical question then is when that state of affairs persists, for how long, and whether it's in force when you want to spend the money on something other than investing. So sure, if you used a short-term time machine to know in advance which volatile stocks are the good ones today, then it would be more profitable to day-trade those than it would be to invest for the long term. Investing on the assumption that you'll only pick short-term winners is basically the same as assuming you have that time machine ;-) There are various strategies for analysing the market and trying to find ways to more modestly do what market makers do, which is to take profit from the inherent volatility of the market. The simple strategy you describe isn't complete and cannot be assessed since you don't say how to decide what to buy, but the selling strategy ""sell as soon as I've made X% but not otherwise"" can certainly be improved. If you're keen you can test a give strategy for yourself using historical share price data (or current share price data: run an imaginary account and see how you're doing in 12 months). When using historical data you have to be realistic about how you'd choose what stocks to buy each day, or else you're just cheating at solitaire. When using current data you have to beware that there might not be a major market slump in the next 12 months, in which case you won't know how your strategy performs under conditions that it inevitably will meet eventually if you run it for real. You also have to be sure in either case to factor in the transaction costs you'd be paying, and the fact that you're buying at the offer price and selling at the bid price, you can't trade at the headline mid-market price. Finally, you have to consider that to do pure technical analysis as an individual, you are in effect competing against a bank that's camped on top of the exchange to get fastest possible access to trade, it has a supercomputer and a team of whizz-kids, and it's trying to find and extract the same opportunities you are. This is not to say the plucky underdog can't do well, but there are systematic reasons not to just assume you will. So folks investing for their retirement generally prefer a low-risk strategy that plays the averages and settles for taking long-term trends." Short answer: Liquidity. Well, you have to see it from an exchange's point of view.  Every contract they put up is a liability to them.  You have to allocate resources for the order book, the matching engine, the clearing, etc.  But only if the contract is actually trading they start earning (the big) money. Now for every new expiry they engage a long term commitment and it might take years for an option chain to be widely accepted (and hence before they're profitable).  Compare the volumes and open interests of big chains versus the weeklies and you'll find that weeklies can still be considered illiquid compared to their monthly cousins. Having said that, like many things, this is just a question of demand.  If there's a strong urge to trade July weeklies one day, there will be an option chain.  But, personally I think, as long as there are the summer doldrums there will be no rush to ask for Jul and Aug chains. Long term gov't bonds fluctuate in price with a seemingly small interest rate fluctuation because many years of cash inflows are discounted at low rates.  This phenomenon is dulled in a high interest rate environment. For example, just the principal repayment is worth ~1/3, P * 1/(1+4%)^30, what it will be in 30 years at 4% while an overnight loan paying an unrealistic 4% is worth essentially the same as the principal, P * 1/(1+4%)^(1/365). This is more profound in low interest rate economies because, taking the countries undergoing the present misfortune, one can see that their overnight interest rates are double US long term rates while their long term rates are nearly 10x as large as US long term rates.  If there were much supply at the longer maturities which have been restrained by interest rates only manageable by the highly skilled or highly risky, a 4% increase on a 30% bond is only about a 20% decline in bond price while a 4% increase on a 4% bond is a 50% decrease. The easiest long term bond to manipulate quantitatively is the perpetuity where p is the price of the bond, i is the interest payment per some arbitrary period usually 1 year, and r is the interest rate paid per some arbitrary period usually 1 year.  Since they are expressly linked, a price can be implied for a given interest rate and vice versa if the interest payment is known or assumed. At a 4% interest rate, the price is At 4.04%, the price is , a 1% increase in interest rates and a 0.8% decrease in price . Longer term bonds such as a 30 year or 20 year bond will not see as extreme price movements. The constant maturity 30 year treasury has fluctuated between 5% and 2.5% to ~3.75% now from before the Great Recession til now, so prices will have more or less doubled and then reduced because bond prices are inversely proportional to interest rates as generally shown above. At shorter maturities, this phenomenon is negligible because future cash inflows are being discounted by such a low amount.  The one month bill rarely moves in price beyond the bid/ask spread during expansion but can be expected to collapse before a recession and rebound during. The dynamics of different contracts and liquidity can be quite different on the last day on the month and for intraday trade make sure you use bid-ask data as opposed to historical trades.  I'm not saying whether it works or not, but im just giving you ideas to improve your testing.