For the US, we offer real-time data from two different sources.
Official real-time data comes directly from the NYSE and Nasdaq exchanges, and comes with exchange fees too. This is the “official” data for a particular stock. If you place a typical trade order with your broker, they will execute the trade using those prices.
Our BATS real-time data comes from the BATS exchange. BATS (which stands for “Better Alternate Trading System”) is a growing electronic exchange based in Kansas City that provides all of its real-time trading data for free. Their data is often very similar to the “official” data from the exchanges, but there can be differences in the data that you should be aware of.
So what exactly are those differences?
Because official non-real-time data from the exchanges is delayed by 15 minutes, we only use BATS data to fill in the most recent 15 minutes on our charts. Once the data is 15 minutes old, we replace that BATS price-only bar with the delayed price+volume bar from the NYSE/Nasdaq. You can see that process here:
This example shows a 1-minute price chart that a typical member using BATS data would see. It shows how we highlight the BATS price-only bars in yellow (because they are subject to change) and how we suppress the BATS volume data as well as volume-based indicators. None of those restrictions happen for members using official real-time data.
Below are two more examples that show how BATS data and NYSE/Nasdaq data differ. The first one shows the differences for a fairly liquid stock (INTC). The BATS version is on the left and the NYSE/Nasdaq version is on the right.
And here's a similar side-by-side comparison for a more thinly-traded stock (EW):
If you look carefully, you'll see that the price bars for INTC match pretty closely (but not exactly) and that the price bars for EW have lots of differences.
If those price differences are important to your analysis or if you don't like the 15-minute delay on the volume data (or if you need real-time data for non-US markets) then you'll want to use official real-time data. Otherwise, our free BATS real-time data should work well for you.