Backtest Overfitting

Testing many strategies on the same data guarantees finding one that looks profitable by chance. With 100 independent trials at p < 0.05, you expect five false positives.

The Problem

Every parameter you tune, every feature you try, and every universe filter you adjust is an implicit trial. A researcher who reports a Sharpe ratio of 2.0 after exploring 200 configurations has not found alpha - they have found the luckiest draw from a noise distribution. Correcting for the number of trials, by haircut here and by Deflated Sharpe Ratio in ml4t-deflated-sharpe, is what separates the two. Without it, most published backtests are statistically meaningless.

The Pattern

WRONG