Before you trust a backtest.

• Walk-forward validation, and why one split can lie.

  Validation

  Choose on the past, score on what came after, and only believe the second number. How it works, how Finertia runs it, and the case where it still misleads.

• How to tell if a backtest is overfit.

  Validation

  Every rule looks good on the years it was tuned on. Four checks that ask whether yours learned a pattern or memorised the noise.

• Lookahead bias: the one-day shift that invents returns.

  Pitfalls

  The most common backtesting bug is one missing line, and nothing warns you. What it is, the quieter forms, and how to check your own results.

• The Sharpe ratio: what it measures and when it misleads.

  Metrics

  Return per unit of risk is the most quoted number in a backtest. What a good one looks like, and four ways it flatters a strategy.

• The permutation test: skill, or just time in the market?

  Validation

  Shuffle a strategy's trades and see whether the real timing beats random timing. How it works, how to read it, and what it cannot tell you.

The rules themselves are on Strategies, and every formula is on How it works.