**Sortino Ratio**

A variant of the Sharpe ratio that only penalises *downside* volatility, ignoring upside swings.

`Sortino = (mean return − risk-free rate) / downside deviation`

Unlike Sharpe, a strategy that has high positive volatility (large wins) is not penalised. This makes Sortino a more useful measure for strategies with asymmetric return distributions.

Higher is better; the same rough thresholds as Sharpe apply (>1 acceptable, >2 good, >3 excellent).
