How Is it Useful?
The Sortino ratio addresses a shortcoming of using standard deviation as a measure of risk in a return-versus-risk trade-off ratio. Standard deviation punishes a manager equally for “good” risk and “bad” risk. Downside deviation adjusts for this by only counting the “bad” risk and ignoring “good” observations in a return series. The Sortino ratio replaces standard deviation with downside deviation, so it is the added return per unit of “bad” risk rather than overall risk.