Sortino Ratio
The Sortino ratio is excess return over downside deviation — Sharpe that only punishes the left side of the return distribution.
Definition
Sortino Ratio refers to sharpe that only punishes the left side of the return distribution. Keep that definition fixed when comparing series, managers, or regimes — renaming the same tape does not create a new signal.
Why it matters
It shows up in factor research, attribution, and capacity debates — whether a return slice is skill, style, or fee drag. When sharpe that only punishes the left side of the return distribution shifts, related hedges, limits, and narratives usually need an explicit update rather than a quiet assumption.
Case
Suppose a desk is positioned for the opposite of what sortino ratio is saying. If sharpe that only punishes the left side of the return distribution moves against that book, the first question is not “is the story clever?” but whether size, hedges, and stop logic still match the observation.
How to read it
Check definition stability across universes, costs, and regimes before treating a backtest as portable. Prefer a short written null hypothesis for Sortino Ratio: what would falsify the current reading in the next window?