When you buy cheap stocks, you should consider that they may be on their way to becomming free (i.e. non-survivors). If your database does not contain stocks that are no longer traded (because the company merged or went out of business), your test results will be unrealistically optimistic.Quote from Blue_Bull:
Can you expand on the survivorship + lookahead biases?
Lookahead bias occurs any time you use data that was unavailable at the time you would have traded. Quarterly and annual financial reports are never available at the end of the querter/year, and they often get restated later. Restatements never make the company/stock look better than before. You need to know when the data was first available and what it looked like then.
