survivorship bias
Survivorship Bias: The 2% Error in Every S&P 500 Backtest
We ran the same value screen on the S&P 500 twice: once with today's members, once with historical membership. Survivorship bias inflated CAGR by 2.02% and hid 7.8 percentage points of drawdown over 25 years. The risk distortion is worse than the return distortion.