This has probably been answered in the past, but if so, I can't find it.

Each evening when I update price data from Fidelity, the screen indicates that data for many of the symbols have been corrected -- perhaps as many as 1/4th of them.

If that much of the price data was incorrect to begin with, can we have much confidence that alerts generated by our strategies based on today's closing data are based on accurate figures? Similarly, backtesting results are obviously based on corrected data in many instances -- and that corrected data may not have been available on the days trades were generated.

Is the answer that we should wait until just before the market opens before updating the data set? Would the corrections have been made by that time? btw, I find that if I update at night and then try to do it again the next morning, nothing new happens.

As I understand it the Fidelity market data group runs a routine each evening that applies data corrections to the most-recent session's data. Most of the corrections are official closing prices. In the past, the routine was completed by 6:00am ET, but there was an effort to complete it much earlier. See the Note in the Data Manager > Update Data > Hour of Update

QUOTE:
I find that if I update at night and then try to do it again the next morning, nothing new happens.

Good point. After you update Fidelity data for a particular day - like before the corrections are available - it won't even try to update those symbols again until after 4pm for the next market session. However, when you update the next day, corrections can be applied for as many as the previous 3 days.

Long story short, it's best to schedule updates for 6am or later to ensure you get the corrections the first time.

Thanks for the prompt reply, Cone.