Standard deviation

Standard deviation measures volatility: how far a fund’s returns stray from their own average. A low number means steady, predictable returns, and a high number means wild swings.

What it tells you

Every fund has an average return, but the average hides the journey. Standard deviation captures that journey by measuring how widely the actual returns scatter around their average. A debt or liquid fund has a low standard deviation, its returns barely move; a small-cap equity fund has a high one, lurching up and down. Two funds can share the same average return while one was calm and the other a roller-coaster, and standard deviation is what tells them apart.

Why it matters for you

The practical point is not the maths but the holding. A high-volatility fund is harder to stay invested in, because the deep drops tempt you to sell at exactly the wrong time. For money you will need sooner, or if steep falls would rattle you, a lower standard deviation is usually worth a slightly lower return. It is the number that turns “risk” from a vague word into something you can compare.

Using it well

Compare standard deviation within a category, since different asset types naturally swing differently. It also feeds the Sharpe ratio, which weighs return against this volatility. Like all such measures it is based on past data, so treat it as a guide to a fund’s character, not a forecast. See risk and return.