Downside Deviation
Also called: downside risk, semi-deviation
Downside deviation measures only the variation below a chosen threshold, ignoring upside swings entirely. It answers the objection that standard deviation punishes a manager for making unexpectedly large gains. It's the denominator of the Sortino ratio, and it separates two portfolios that look identical on volatility but differ completely in shape — one that drifts up and crashes, and one that grinds steadily.
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