survivorship bias comes into play when looking at historical returns for a given index, for example if you are using 'stocks in the s&p 500' as your dataset to backtest against, you are chosing the stocks that today are worth a lot... but in 2001, there were some stocks in the s&p 500 that are no longer in that index... they did not survive. same goes for using 'all stocks' because that dataset similarly doesn't include any companies that disappeared btw 2001 and today... if you want to backtest against the 2001-2015 time period, you'll want to chose which stocks per backtested day to look at based on their rank in some index on that day... to eliminate survivorship bias.