Downside risk captures only the negative side of volatility — the risk of losses rather than the total variability of returns (both up and down).
Standard deviation (the basis of Sharpe Ratio) treats all volatility equally — it penalises a fund with large positive surprises the same as one with large negative surprises. Downside risk focuses only on the loss side: periods when returns fell below a target or threshold. It is quantified by semi-deviation (standard deviation of below-target returns) and is the foundation for Sortino Ratio. For investors, downside risk is the more emotionally and financially relevant risk measure.
You don't lose sleep when your fund is up 30% unexpectedly. You lose sleep when it drops 20%. Downside risk measures only that second scenario — the magnitude and frequency of negative surprises. A fund that swings wildly upward but rarely falls is high-volatility but low downside risk. Downside risk is what actually affects your financial plan and emotional wellbeing.
Identify a target/minimum return (e.g., 0%, risk-free rate).
Calculate returns that fall below this target.
Downside deviation = standard deviation of only those below-target return periods.
Used in Sortino Ratio: excess return ÷ downside deviation.