I'm backtesting an EOD system on stocks that enters and exits on open and close. I'm wondering how accurate my historical opening and closing prices are in relation to what really happens? I've always wondered how the exact price can be determined, since many trades happen in those split seconds. Which ones are actually reported as open and close? I understand too that the rules are different on the NASDAQ where the market is distributed, and the NYSE, where it is more centralized. Can anyone comment on what's behind these prices?