“Inside these data feeds is information which allows folks to read it and re-engineer the behavior of others,” Saluzzi said. “A lot of high-frequency strategies are built on modeling the behaviors of someone else.”
The private feeds also reach traders more quickly than the public-quote system because they are sent directly from each exchange to paying customers. Public feeds build in an additional step: Price data from dozens of venues where U.S. stocks change hands are sent to a central place for processing before that information is publicized. Bloomberg LP, the parent of Bloomberg News, provides its clients with access to some proprietary exchange feeds.
A study published in January co-authored by Terrence Hendershott, associate professor of finance at the University of California, Berkeley, found the average time difference was 1.5 milliseconds between calculating a stock’s price using the exchange’s proprietary data and waiting for the public information. That’s more than enough time for a speedy trader to recognize an advantageous price and execute a trade against someone using the slower feed.