Downside Deviation (DD) measures the volatility of returns that fall below a target (Minimum Acceptable Return, MAR) — most commonly the risk-free rate or 0%. Unlike standard deviation it ignores upside variation, matching how investors actually feel risk. It is the denominator of the Sortino Ratio.
Worked INR example
A flexi-cap fund's monthly returns over 36 months show standard deviation 14% (annualised). Counting only months where return fell below 0.54% (6.5% risk-free / 12), the downside deviation = 8.5%. The fund's upside vol (which doesn't hurt) was 11.5%. Sortino = excess return / 8.5% vs Sharpe = excess return / 14% — Sortino is materially higher, showing the fund's positive skew.
When to use
- Equity / hybrid funds with right-skewed returns where SD overstates risk
- Goal-based investing — set MAR = required CAGR for the goal
- Option-selling strategies where upside is capped but downside is fat-tailed
SEBI caveat
Choice of MAR (0%, risk-free, benchmark) materially changes DD — always disclose. SEBI factsheets don't show DD; compute from monthly returns. Sample-size: minimum 36 months, ideally 60.
Related terms: Downside Deviation, Sortino Ratio, Standard Deviation.