Strange values in ARM.L price data 1998-2000 from Yahoo
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.
"At the bottom of Yahoo! Finance's S & P 500 quote Quotes are real-time for NASDAQ, NYSE, and NYSE MKT. See also delay   times for other exchanges. All information provided ""as is"" for   informational purposes only, not intended for trading purposes or   advice. Neither Yahoo! nor any of independent providers is liable for   any informational errors, incompleteness, or delays, or for any   actions taken in reliance on information contained herein. By   accessing the Yahoo! site, you agree not to redistribute the   information found therein. Fundamental company data provided by Capital IQ. Historical chart data   and daily updates provided by Commodity Systems, Inc. (CSI).   International historical chart data, daily updates, fund summary, fund   performance, dividend data and Morningstar Index data provided by   Morningstar, Inc. Orderbook quotes are provided by BATS Exchange. US   Financials data provided by Edgar Online and all other Financials   provided by Capital IQ. International historical chart data, daily   updates, fundAnalyst estimates data provided by Thomson Financial   Network. All data povided by Thomson Financial Network is based solely   upon research information provided by third party analysts. Yahoo! has   not reviewed, and in no way endorses the validity of such data. Yahoo!   and ThomsonFN shall not be liable for any actions taken in reliance   thereon. Thus, yes there is a DB being accessed that there is likely an agreement between Yahoo! and the providers." There are companies who sell data gleaned in aggregate from credit card providers to show how much of what category of product is sold online or offline, but that data is not cheap.  1010data is one such data aggregator we are talking to right now. Haven’t seen pricing yet but I expect 6 figures to access the data It is because 17th was Friday, 18th-19th were weekends and 20th was a holiday on the Toronto Stock Exchange (Family Day). Just to confirm you could have picked up another stock trading on TMX and observed the price movements. The 15% Alibaba stake was invested in way before Mayer's came on board.  It's worth $51.75B right now.  Yahoo Japan is an autonomous company, that Yahoo has a 36% stake in, spun off well before Mayer's came on board and that stake is conservatively worth$10B.  Yahoo market cap is only $49B right now.  If anything, Marissa Mayers tenure just killed off the core value of Yahoo itself to the level that it's a liability and it's asset portfolio is the only thing holding it aloft. "For the S&P and many other indices (but not the DJIA) the index ""price"" is just a unitless number that is the result of a complicated formula.  It's not a dollar value.  So when you divide said number by the earnings/share of the sector, you're again getting just a unitless number that is incomparable to standard P-E ratios. In fact, now that I think about, it kinda makes sense that each sector would have a similar value for the number that you're computing, since each sector's index formula is presumably written to make all the index ""price""s look similar to consumers." Two reasons why I think that's irrelevant: First, if it was on 3/31/2012 (two other sources say it was actually 4/3/2012), why the big jump two trading days later? Second, the stock popped up from $3.10 to $4.11, then over the next several trading days fell right back to $3.12. If this were about the intrinsic value of the company, I'd expect the stock to retain some value. I don't see it in any of the exchange feeds I've gone through, including the SIPs. Not sure if there's something wrong with Nasdaq Last Sale (I don't have that feed) but it should be putting out the exact same data as ITCH. Apparent data-feed issues coming out of NASDAQ in the after hours market. Look at MSFT, AMZN, AAPL, heck even Sears.      Funny thing though, is that you see traces of irregular prices during the active session around 10:20am on stocks like GOOG. Look at the 'as of'.  Google's as of is 11:27 whil Yahoo's is 11:19.  Given the shape of the Google curve, it looks to me that Yahoo's may well drop that much in the next 8 minutes. In fact, looking at it now, Yahoo's algorithm showed it as about 30 at 11:24, before going back up again some.  It may not have been identical to Google's, but it was certainly close. "Remember that in most news outlets journalists do not get to pick the titles of their articles.  That's up to the editor.  So even though the article was primarily about ETFs, the reporter made the mistake of including some tangential references to mutual funds.  The editor then saw that the article talked about ETFs and mutual funds and -- knowing even less about the subject matter than the reporter, but recognizing that more readers' eyeballs would be attracted to a headline about mutual funds than to a headline about ETFs -- went with the ""shocking"" headline about the former. In any case, as you already pointed out, ETFs need to know their value throughout the day, as do the investors in that ETF.  Even momentary outages of price sources can be disastrous. Although mutual funds do not generally make transactions throughout the day, and fund investors are not typically interested in the fund's NAV more than once per day, the fund managers don't just sit around all day doing nothing and then press a couple buttons before the market closes.  They do watch their NAV very closely during the day and think very carefully about which buttons to press at the end of the day.  If their source of stock price data goes offline, then they're impacted almost as severely as -- if less visibly than -- an ETF. Asking Yahoo for prices seems straightforward, but (1) you get what you pay for, and (2) these fund companies are built on massive automated infrastructures that expect to receive their data from a certain source in a certain way at a certain time.  (And they pay a lot of money in order to be able to expect that.)  It would be quite difficult to just feed in manual data, although in the end I suspect some of these companies did just that.  Either they fell back to a secondary data supplier, or they manually constructed datasets for their programs to consume." The point of that question is to test if the user can connect shares and stock price.  However, that being said yeah, you're right. Probably gives off the impression that it's a bit elementary. I'll look into changing it asap. A number of places.  First, fast and cheap, you can probably get this from EODData.com, as part of a historical index price download -- they have good customer service in my experience and will likely confirm it for you before you buy.  Any number of other providers can get it for you too.  Likely Capital IQ, Bloomberg, and other professional solutions. I checked a number of free sites, and Market Watch was the only that had a longer history than a few months. Am I missing something or is the author?  EUP5  USP5  SUP5  PUS5  EDIT: Its not just currency pairs either, 5DEL; EU5L; 5UKL; EU5S; 5DES.  Unless I misunderstood something, the authors worst nightmare has already arrived in Europe. "Thanks for the response.   I am using WRDS database and we are currently filtering through various variables like operating income, free cash flow etc.  Main issue right now is that the database seems to only go up to 2015...is there a similar database that has 2016 info?  filtering out the ""recent equity issuance or M&amp;A activity exceeding 10% of total assets"" is another story, namely, how can I identify M&amp;A activity? I suppose we can filter it with algorithm stating if company's equity suddenly jumps 10% or more, it get's flagged" I work on a buy-side firm, so I know how these small data issues can drive us crazy. Hope my answer below can help you:  Reason for price difference: 1. Vendor and data source Basically, data providers such as Google and Yahoo redistribute EOD data by aggregating data from their vendors. Although the raw data is taken from the same exchanges, different vendors tend to collect them through different trading platforms. For example, Yahoo, is getting stock data from Hemscott (which was acquired by Morningstar), which is not the most accurate source of EOD stocks. Google gets data from Deutsche Börse.  To make the process more complicated, each vendor can choose to get EOD data from another EOD data provider or the exchange itself, or they can produce their own open, high, low, close and volume from the actual trade tick-data, and these data may come from any exchanges.  2. Price Adjustment  For equities data, the re-distributor usually adjusts the raw data by applying certain customized procedures. This includes adjustment for corporate actions, such as dividends and splits. For futures data, rolling is required, and back-ward and for-warding rolling can be chosen. Different adjustment methods can lead to different price display.  3. Extended trading hours  Along with the growth of electronic trading, many market tends to trade during extended hours, such as pre-open and post-close trading periods. Futures and FX markets even trade around the clock. This leads to another freedom in price reporting: whether to include the price movement during the extended trading hours.   Conclusion To cross-verify the true price, we should always check the price from the Exchange where the asset is actually traded. Given the convenience of getting EOD data nowadays, this task should be easy to achieve.  In fact, for professional traders and investors alike, they will never reply price on free providers such as Yahoo and Google, they will most likely choose Bloomberg, Reuters, etc. However, for personal use, Yahoo and Google should both be good choices, and the difference is small enough to ignore.