Glossary

Downside Deviation

Also known as: Semi-Deviation

A volatility measure that calculates the dispersion of only those returns falling below a specified minimum acceptable return, often zero or a risk-free benchmark, rather than the dispersion of all returns above and below the mean as in standard deviation.

Standard deviation penalizes upside volatility (unexpectedly strong returns) exactly as heavily as downside volatility, which misrepresents how investors actually experience risk, since most investors have no aversion to positive surprises; downside deviation corrects for this by measuring dispersion only among below-target outcomes, and serves as the risk denominator in the Sortino ratio in the same way standard deviation serves as the denominator in the Sharpe ratio. The distinction is particularly meaningful for real estate portfolios, whose already appraisal-smoothed return series understate true return dispersion in either direction, making the choice between a downside-only and a full-distribution risk measure a substantive rather than purely academic decision when comparing managers with asymmetric return profiles, such as a development-heavy strategy with capped downside but open-ended upside.