Downside Deviation

Unlike regular volatility, which counts every swing up or down, downside deviation only measures the bad ones. A fund that swings wildly on good days but rarely loses money will show high standard deviation but low downside deviation — it isolates the risk of actually losing money.

Downside deviation modifies the standard volatility formula to only count returns that fall below a chosen target (often zero, or the risk-free rate), treating any return above the target as zero deviation. This isolates 'bad' volatility from total volatility — a strategy with large upside swings but rare, small losses will show high standard deviation but low downside deviation. It's the denominator in the Sortino ratio, which many practitioners view as fairer than the Sharpe ratio for strategies with asymmetric return distributions, since Sharpe penalizes upside volatility exactly the same as downside volatility. The target return chosen materially changes the number, so check what minimum acceptable return (MAR) was used before comparing downside deviation across two funds.

DD = √(Σ min(Ri - Target, 0)² ÷ n)