Sortino Ratio.
A definition, in plain English — with the books that teach it.
What it means
The Sortino Ratio refines the Sharpe Ratio by dividing excess return only by downside deviation — the standard deviation of negative returns — rather than total volatility. This distinction matters because investors typically dislike losses but welcome upside swings. A strategy with frequent large gains and rare small losses can look worse on Sharpe than it actually is; the Sortino Ratio corrects for that by ignoring volatility that works in the investor's favor.
Example
A fund returned 11% with a downside deviation of 6% and the risk-free rate was 4%. Sortino Ratio = (11% − 4%) / 6% = 1.17. If total standard deviation was 10%, the Sharpe would be only 0.70 — the Sortino reveals that most of the fund's volatility was on the upside.
