Do query services like Google Finance and Yahoo Finance go back to correct busted and adjusted trades?
No. Busts are very infrequent, and if an equity were illiquid enough to be affected, the bust cost would be enormous. For a liquid equity, the amount of busted volume is insignificant except during a flash crash or flash spike.  Then it would be reasonable to redownload.
Well, kind of XD. I usually just look through Business Week for the ADRs that are on the OTC market. I don't do anything major, but why I love them is that they have a greater reach than just ADRs on the NYSE or NASDAQ. Like for instance, if I wanted to own Thai or European stocks, many of the larger, more reputable firms are listed on the OTC market. Having said that, most other sites don't have earnings reports laid out for you. Business Week does. The only fancy thing I am interested in are options and options on futures, and Bar Chart is good for the latter. This is just a shot in the dark but it could be intermarket data. If the stock is interlisted and traded on another market exchange that day then the Yahoo Finance data feed might have picked up the data from another market. You'd have to ask Yahoo to explain and they'd have to check their data. "Okay, so you don't know what other options are available, understood. No need to be a dick. In other industries, deals and transactions are invalidated, reversed, refunded or other measures taken all the time when they're based on invalid information. Hence, why I was asking what options there are for these situations. I know that if I saw CSCO go down 40% and the info scanned on official sources that I would dump a ton of money into buying it up if I had it available. I mean, hell, it's Cisco Systems who practically owns the internet. They'll be back up from that hit and fairly quickly. To find out that was all just because some over-worked programmer put a comma where there should have been a period would have me on the warpath if trend was actually the other way. With the amount of money that flows through such a system, there has to be some kind of process for dealing with these things. If you don't know, then no worries. There's no need to jump on me for asking a question when a simple ""I don't know"" would suffice." "Often these types of trades fall into two different categories.   An error by broker or exchange.  Exchange clearing out part of their books incorrectly is an example.  Most exchanges make firms reopen their positions for after market hours.  There may have been an issue doing so or exchange could incorrectly cancel positions.  I was in the direct feed industry for years and this was a big issue.  At the same time the broker can issue a no limit buy on accident (or has software that is prospecting and said software has a bug or written poorly).   unscrupulous parties looking to feign an upswing or downswing in market.  Let's say you hold 500k shares in a stock that sells for $11.  You could possibly buy 100 shares for $13.  Trust me you will find a seller.  Then you are hoping that people see that trade as a ""norm"" and trade from there, allowing you to rake in $1M for spending an extra $200 - NOTE this is not normal and an extreme example.  This was so common in the early days of NASDAQ after hours that they discontinued using the after hours trades as part of historical information that they keep like daily/yearly high or closing price.  The liquidity allows for manipulation.  It isn't seen as much now since this has been done a million times but it does still happen." I still can't understand why there is a price discrepancancy.  There isn't. It's the same stock and price differences between such major exchanges will always be minimal. I think you simply haven't paid attention to the date range. It seems Google finance only has data for FRA:BMW reaching back to 2011, so if you try to look at the development of your investment since 2009, you're not getting comparable data. I should have been clearer but my point was that the NYSE seems to be blaming third party vendors for reporting invalid test data but their own website reported the same data so it seems like there might be another issue.   Edit: Found the full comment. It seems that NASDAQ distributed the test data and other parties including the NYSE incorrectly displayed it. I can (barely) understand some third parties incorrectly reporting this data but it seems really bizarre that NYSE wouldn't know how to handle this. The yahoo finance API is no longer which broke the Finance:Quote perl module.  The Finance:Quote developers have been quick to fix things and have produced several new versions in the last week or two.  The short of it is that you need to update Finance:Quote, then obtain an AlphaVantage free key and tell Gnucash to use AlphaVantage as it's source for online quotes by editing your securities in the Price Editor. There usually would be a SEC investigation if this kind of behavior is noticed. One thing that bothers me about this article is that, in the midst of all the fear mongering, the author doesn't mention that quote stuffing is designed to screw over *other HFT algorithms* and doesn't have any real impact on investors. Firms doing HFT frequently report this stuff to the SEC; for all we know, Goldman already did. On Monday, the 27th of June 2011, the XIV ETF underwent a 10:1 share split. The Yahoo Finance data correctly shows the historic price data adjusted for this split.  The Google Finance data does not make the adjustment to the historical data, so it looks like the prices on Google Finance prior to 27 June 2011 are being quoted at 10 times what they should be.   Coincidentally, the underlying VIX index saw a sudden surge on the Friday (24 June) and continued on the Monday (27 June), the date that the split took effect.  This would have magnified the bearish moves seen in the historic price data on the XIV ETF. Here is a link to an article detailing the confusion this particular share split caused amongst investors.  It appears that Google Finance was not the only one to bugger it up.  Some brokers failed to adjust their data causing a lots of confusion amongst clients with XIV holdings at the time. This is a recurring problem on Google Finance, where the historic price data often (though not always) fails to account for share splits. The biggest issue with Yahoo Finance is the recent change to the API in May. The data is good quality, includes both dividend/split adjusted and raw prices, but it's much more difficult to pull the data with packages like R quantmod than before.  Google is fine as well, but there are some missing data points and you can't unadjust the prices (or is it that they're all unadjusted and you can't get adjusted? I can't recall). I use Google at home, when I can't pull from Bloomberg directly and when I'm not too concerned with accuracy.  Quandl seems quite good but I haven't tried them. There's also a newer website called www.alphavantage.co, I haven't tried them yet either but their data seems to be pretty good quality from what I've heard. "I'm not sure which article you are referring to- if you are talking about the convergex article.... it literally describes what they did in the article.   Tradeworx and software company thesys(both founded by manoj narang) provide data to the sec and function as market makes- they are highly respected and handle over 5% of us equity by volume.   Their ""head people"" is manoj narang. I don't think he ""bailed out"". As you can read [here](https://research-doc.credit-suisse.com/docView?sourceid=em&amp;document_id=x569866&amp;serialid=lZPbU6l0cgAqB%2B1gg4uZFLk14dBwhfSb9lZ3%2BdmPHV4%3D) they are willing 53% of the trades they place everyday and have   He has stated in an interview he wants to explore options trading.   They were debunking myths because of the regulators wanted answers and at the time hft was not understood very well. Narang also testified at a senate committee[video here](https://youtu.be/NG47K41Q7KA)" First - Google's snapshot -  Then - Yahoo -  I took these snapshots because they will not exist on line after the market opens, and without this context, your question won't make sense. With the two snapshots you can see, Yahoo shows the after hours trades and not just the official market close for the day. The amount it's down is exactly tracked from the close shown on Google. Now you know. I had the same problem and was looking for a software that would give me easy access to historical financial statements of a company, preferably in a chart. So that I could easily compare earnings per share or other data between competitors. Have a look at Stockdance this might be what you are looking for.  Reuters Terminal is way out of my league (price and complexity) and Yahoo and Google Finance just don't offer the features I want, especially on financials. Stockdance offers a sort of stock selection check list on which you can define your own criterion’s. Hence it makes no investment suggestions but let's you implement your own investing strategy. It looks like GOOG did not have a pre-market trade until 7:14 am ET, so Google Finance was still reporting the last trade it had, which was in the after-hours session yesterday. FB, on the other hand, was trading like crazy after-hours yesterday and pre-market today as it had an earnings report yesterday. No. Like Keshlam said, unless you have a crystal ball there is no sure thing. However based on the things you said in your question, you could be better off doing some back testing. With your findings, you can then set up trades in your favor but again it's not 100%. You may also want to check out quant finance stackexhange.