Sortino Ratio
A Sharpe variant that divides excess return only by downside deviation, ignoring upside volatility as a "risk".
Sortino Ratio = (Strategy Return − Risk-Free Rate) / Downside Deviation
The Sortino Ratio improves on the Sharpe Ratio by using only downside standard deviation — the volatility of negative returns — as the risk denominator. This avoids penalising strategies for large up-moves, which are desirable.
For strategies with asymmetric return distributions (large winners, small losers), the Sortino Ratio gives a much more flattering and accurate picture of risk-adjusted performance than Sharpe.