Where do traders take their prices data from? How can it be different from their brokers'?
"This is a complicated subject, because professional traders don't rely on brokers for stock quotes.  They have access to market data using Level II terminals, which show them all of the prices (buy and sell) for a given stock.   Every publicly traded stock (at least in the U.S.) relies on firms called ""market makers"".  Market makers are the ones who ultimately actually buy and sell the shares of companies, making their money on the difference between what they bought the stock at and what they can sell it for.  Sometimes those margins can be in hundreds of a cent per share, but if you trade enough shares...well, it adds up. The most widely traded stocks (Apple, Microsoft, BP, etc) may have hundreds of market makers who are willing to handle share trades.  Each market maker sets their own price on what they'll pay (the ""bid"") to buy someone's stock who wants to sell and what they'll sell (the ""ask"") that share for to someone who wants to buy it.   When a market maker wants to be competitive, he may price his bid/ask pretty aggressively, because automated trading systems are designed to seek out the best bid/ask prices for their trade executions.  As such, you might get a huge chunk of market makers in a popular stock to all set their prices almost identically to one another.  Other market makers who aren't as enthusiastic will set less competitive prices, so they don't get much (maybe no) business.  In any case, what you see when you pull up a stock quote is called the ""best bid/ask"" price.  In other words, you're seeing the highest price a market maker will pay to buy that stock, and the lowest price that a market maker will sell that stock.  You may get a best bid from one market maker and a best ask from a different one.  In any case, consumers must be given best bid/ask prices. Market makers actually control the prices of shares.  They can see what's out there in terms of what people want to buy or sell, and they modify their prices accordingly.  If they see a bunch of sell orders coming into the system, they'll start dropping prices, and if people are in a buying mood then they'll raise prices. Market makers can actually ignore requests for trades (whether buy or sell) if they choose to, and sometimes they do, which is why a limit order (a request to buy/sell a stock at a specific price, regardless of its current actual price) that someone places may go unfilled and die at the end of the trading session.  No market maker is willing to fill the order. Nowadays, these systems are largely automated, so they operate according to complex rules defined by their owners.  Very few trades actually involve human intervention, because people can't digest the information at a fast enough pace to keep up with automated platforms. So that's the basics of how share prices work.  I hope this answered your question without being too confusing! Good luck!" To add a bit to Daniel Anderson's great answer, if you want to 'peek' at what a the set of bid and ask spreads looks like, the otc market page could be interesting (NOTE:  I'm NOT recommending that you trade Over The Counter.  Many of these stocks are amusingly scary):  http://www.otcmarkets.com/stock/ACBFF/quote You can see market makers essentially offering to buy or sell blocks of stock at a variety of prices.
You can get this data from a variety of sources, but likely not all from 1 source.  Yahoo is a good source, as is Google, but some stock markets also give away some of this data, and there's foreign websites which provide data for foreign exchanges.    Some Googling is required, as is knowledge of web scraping (R, Python, Ruby or Perl are great tools for this...). Bloomberg is very popular, especially for researching individual companies. Market QA and Factset are popular for analyzing data. Microsoft Excel and Matlab are very common for analyzing the data.  Lots of time traders will take data from Bloomberg, Market QA, Factset or where ever, and then actually preform their analytics in Excel or Matlab. A lot of the brokers provide their own software to traders using their platform, and that software can also contain different tools to help the traders as well. A day is a long time and the rate is not the same all day. Some sources will report a close price that averages the bid and ask. Some sources will report a volume-weighted average. Some will report the last transaction price. Some will report a time-weighted average. Some will average the highest and lowest prices for the interval. Different marketplaces will also have slightly different prices because different traders are present at each marketplace. Usually, the documentation will explain what method they use and you can choose the source whose method makes the most sense for your application. To dig a little deeper, a number of analysts within (and without) Reuters are polled for their views on individual stocks and markets on buy-hold-sell. The individual analysts will be a varied bunch of fundamentalists, technical, quant and a mixture of the three plus more arcane methodologies. There may be various levels of rumors that aren't strong enough to be considered insider trading, but all of these will give an analyst an impression of the stock/market. Generally I think there isn't much value there, except from the point of view if you are a contrarian trader, then this will form a part of the input to your trading methodology. While a lot of trading is executed by computers, a substantial amount is still done at the behest of humans.  Brokers managing accounts, Portfolio Managers, and Managers of Mutual Funds doing stock picks etc.  Those folks are still initiating a very large number of the trades (or at least one side of a trade).  And those humans don't work 7 days a week. it's not just computers talking to computers at the behest of other computers.  And even a lot of places that use computers to create models and such, there are still humans in the loop to ensure that the computers are not ordering something stupid to be done. I personally worked for a firm that managed nearly $20Billion in stock portfolios.  The portfolios were designed to track indexes, or a mix of indexes and actively managed funds, but with the addition of managing for tax efficiency.  A lot of complex math and complicated 'solver' programs that figured out each day what if anything to trade in each portfolio.  Despite all those computers, humans still reviewed all the trades to be sure they made sense. And those humans only worked 5 days a week. They don't have to track each other, it could just be listed on more than one exchange.  The price on one exchange does not have to match or track the price on the other exchange. This is actually quite common, as many companies are listed on two or more exchanges around the world. "I may be underestimating your knowledge of how exchanges work; if so, I apologize.  If not, then I believe the answer is relatively straightforward. Lets say price of a stock at time t1 is 15$ .  There are many types of price that an exchange reports to the public (as discussed below); let's say that you're referring to the most recent trade price.  That is, the last time a trade executed between a willing buyer and a willing seller was at $15.00. Lets say a significant buy order of 1M shares came in to the market. Here I believe might be a misunderstanding on your part.  I think you're assuming that the buy order must necessarily be requesting a price of $15.00 because that was the last published price at time t1.  In fact, orders can request any price they want.  It's totally okay for someone to request to buy at $10.00.  Presumably nobody will want to sell to him, but it's still a perfectly valid buy order. But let's continue under the assumptions that at t1: This makes the bid $14.99 and the ask $15.00.  (NYSE also publishes these prices.) There aren't enough people selling that stock. It's quite rare (in major US equities) for anyone to place a buy order that exceeds the total available shares listed for sale at all prices.  What I think you mean is that 1M is larger than the amount of currently-listed sell requests at the ask of $15.00. So say of the 1M only 100,000 had a matching sell order and others are waiting. So this means that there were exactly 100,000 shares waiting to be sold at the ask of $15.00, and that all other sellers currently in the market told NYSE they were only willing to sell for a price of $15.01 or higher.  If there had been more shares available at $15.00, then NYSE would have matched them. This would be a trigger to the automated system to start increasing the price. Here is another point of misunderstanding, I think.  NYSE's automated system does not invent a new, higher price to publish at this point.  Instead it simply reports the last trade price (still $15.00), and now that all of the willing sellers at $15.00 have been matched, NYSE also publishes the new ask price of $15.01.  It's not that NYSE has decided $15.01 is the new price for the stock; it's that $15.01 is now the lowest price at which anyone (known to NYSE) is willing to sell.  If nobody happened to be interested in selling at $15.01 at t1, but there were people interested in selling at $15.02, then the new published ask would be $15.02 instead of $15.01 -- not because NYSE decided it, but just because those happened to be the facts at the time. Similarly, the new bid is most likely now $15.00, assuming the person who placed the order for 1M shares did not cancel the remaining unmatched 900,000 shares of his/her order.  That is, $15.00 is now the highest price at which anyone (known to NYSE) is willing to buy. How much time does the automated system wait to increment the price, the frequency of the price change and by what percentage to increment etc. So I think the answer to all these questions is that the automated system does none of these things.  It merely publishes information about (a) the last trade price, (b) the price that is currently the lowest price at which anyone has expressed a willingness to sell, and (c) the price that is currently the highest price at which anyone has expressed a willingness to buy. ::edit:: Oh, I forgot to answer your primary question. Can we estimate the impact of a large buy order on the share price? Not only can we estimate the impact, but we can know it explicitly.  Because the exchange publishes information on all the orders it knows about, anyone tracking that information can deduce that (in this example) there were exactly 100,000 shares waiting to be purchased at $15.00.  So if a ""large buy order"" of 1M shares comes in at $15.00, then we know that all of the people waiting to sell at $15.00 will be matched, and the new lowest ask price will be $15.01 (or whatever was the next lowest sell price that the exchange had previously published)." There are about 250 trading days in a year. There are also about 1,900 stocks listed on the NYSE.  What you're asking for would require about 6.2M rows of data. Depending on the number of attributes you're likely looking at a couple GB of data.  You're only getting that much information through an API or an FTP. "Using your example link, I found the corresponding chart for a stock that trades on London Stock Exchange: https://ca.finance.yahoo.com/echarts?s=RIO.L#symbol=RIO.L;range=1d As you can see there, the chart runs from ~8:00am to ~4:30pm, and as I write this post it is only 2:14pm Eastern Time.  So clearly this foreign chart is using a foreign time zone.  And as you can see from this Wikipedia page, those hours are exactly the London Stock Exchange's hours. Additionally, the closing price listed above the graph has a timestamp of ""11:35AM EST"", meaning that the rightmost timestamp in the graph (~4:30pm) is equal to 11:35AM EST.  16:30 - 11:30 = 5 hours = difference between London and New York at this time of year. So those are two data points showing that Yahoo uses the exchange's native time zone when displaying these charts." I don't quite understand the NYSE argument that the credit system helps NASDAQ undercut NYSE on pricing and force brokers to trade on NASDAQ. I thought if you were trading a stock listed on NASDAQ, you traded through them and if you were trading a stock listed on NYSE, you'd trade over there. The choice of exchange coming down to the stocks you want to trade more than anything. Are the exchanges also acting as endpoints on trades for securities listed on the other exchange? "Pay to play? You mean they pay for beefier data feeds that other firms don't need, sure they do. But everyone can trade on the US exchanges now (NYSE, NASDAQ, BATS, etc) which was not true 20 years ago when NYSE had the specialist monopoly, so yes the markets are more democratic than they've ever been.  Side note, the exchanges do directly profit from increased trading volume because they take a small % of every trade, so I'm not sure what you mean they don't directly profit from it.  Also I get the feeling you don't understand the scope of HFT activity, HFT peak profits were on the order of 7 billion during the highest volume time of the last decade (2005-2010). They are now around 1B. Compared to the trillions of dollars that change hands in the exchange per year, this is chump change, hardly a ""free money faucet"".  Also Katsayuma opened yet another dark pool that caters to high volume clients (your goldmans and merrils). The only difference in IEX is it got a free marketing campaign to entice clients. Seriously, IEX is nothing more than the existing fixed cross dark pools, which btw screws over retail investors more than the lit exchanges like NASDAQ.   Katsayuma got steamrolled in his executions because he couldn't keep up with the time and then hit the lottery jackpot by getting Michael Lewis to paint him as a ""hero"", honestly I'm confounded at how lucky that dude got.  BTW broker dealers get preferential treatment in IEX, meaning they get to cut in front of the line in front of retail investors. Why are you so opinionated about this, did you make a few bad trades on eTrade and need a scapegoat?" the data source is the same as the live market trading. pre and after market trading are active markets and there are actual buyers and sellers getting their orders matched. "That's like a car dealer advertising their ""huge access"" to Chevrolet.  All brokers utilize dark pools nowadays, either their own or one belonging to a larger financial institution.  Why?  Because that's a primary source of broker income.  Example: Under current US regulations the broker is under no obligation to pass these orders to actual (a.k.a. lit) exchanges.  Instead it can internalize them in its dark pool as long as it ""improves the price"".  So: If a broker doesn't run its own dark pool, then it sends the orders to the dark pool run by a larger institution (JPMorgan, Credit Suisse, Getco, Knight Capital) and gets some fraction of the dark pool's profit in return. Are Mom and Pop negatively impacted by this?  Not for most order types.  They each even got a free penny out of the deal!  But if there were no dark pools, that $1.00 difference between their trade prices would have gone half ($0.50) to Mom's counterparty and half ($0.50) to Pop's counterparty, who could be someone else's Mom and someone else's Pop. So ... that's why brokers all use dark pools, and why their advertisement of their dark pool access is silly.  They're basically saying, ""We're going to occasionally throw you a free penny while making 49 times that much from you""! (Note: Now apply the above math to a less liquid product than AAPL.  Say, where the spread is not $0.01, but more like $0.05.  Now Mom and Pop still might make a penny each, while the broker can make $4.98 on a 100 share trade!)" You might have better luck using Quandl as a source.  They have free databases, you just need to register to access them.  They also have good api's, easier to use than the yahoo api's Their WIKI database of stock prices is curated and things like this are fixed (www.quandl.com/WIKI ), but I'm not sure that covers the London stock exchange.  They do, however, have other databases that cover the London stock exchange. 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.