Downside risk.

In plain English

Downside risk isolates the unwanted half of volatility, measuring how far and how often returns fall under a chosen threshold rather than how much they vary in both directions. Standard deviation treats a big gain and a big loss as equally undesirable, which does not match how anyone actually experiences a portfolio. Downside deviation fixes that by including only the periods that fell short of the target, whether the target is zero, a cash rate, or a required return. It feeds ratios that judge return per unit of loss risk rather than per unit of total movement. It also puts a number on the sequence problem facing someone drawing income, where an early loss does lasting damage.

01Why it matters

Two investments can show the same volatility while one delivers its swings mostly as gains and the other mostly as losses, and only a downside measure separates them.

02The math, step by step

A fund returns plus 12, plus 15, minus 8, plus 10, and minus 6 percent over five years. Total volatility counts all five deviations. Downside deviation against a zero target counts only the minus 8 and the minus 6, giving the square root of ((64 plus 36) divided by 5), or about 4.5 percent.

Illustrative example. The amounts here are hypothetical, chosen to show how the math works, not real quoted rates or figures.

03What this is NOT

04Receipts

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