How to Read a Backtest Without Fooling Yourself: Look-Ahead, Survivorship, Costs and Overfitting
Four errors, each measured on the same data: look-ahead turned a 200-day moving-average rule from 9.4% a year into 20.9%; today's S&P 500 members beat the real equal-weight fund by 6.3 points a year since 2004; 10 bp a trade sank a 45-trade-a-year rule; and the best of 355 moving-average pairs ranked 322nd out of sample.















