Standard deviation
A measure of how widely an investment’s returns have varied around their average; a higher figure means a bumpier ride.
Standard deviation measures volatility — how far individual returns typically stray from their average. A fund with an average annual return of 12% and a standard deviation of 5% has had returns clustered fairly close to 12%. One with the same average and a standard deviation of 20% has seen much wider swings, including years well below zero.
Why it matters
Volatility is what you actually live through as an investor: the falls that test your patience and the rallies that tempt you to add more. A fund’s historical standard deviation helps you judge whether you could stay invested through its typical ups and downs — which matters more to long-term outcomes than almost anything else.
How to read it
- Compare within a category. Equity funds almost always show higher standard deviation than short-term debt funds.
- It is usually annualised from monthly or daily returns over three years or more.
- As a rough guide, if returns followed a normal distribution, about two-thirds of outcomes would fall within one standard deviation of the average. Real markets produce extreme events more often than that suggests.
Common misconceptions
- “Volatility and risk are the same.” Volatility is one dimension. The risk that matters most for a goal is not having the money when you need it, which also depends on horizon, liquidity and concentration.
- “Past volatility sets future limits.” Market crises routinely produce moves larger than historical standard deviation would suggest.
In India: Many equity and hybrid scheme factsheets publish standard deviation alongside beta and the Sharpe ratio.
Formula
Standard deviation = √[Σ (Returnᵢ − Average return)² ÷ (n − 1)]
Worked example
For illustration, assume a fund’s yearly returns over five years were 20%, −5%, 15%, 10% and 10%. The average is 10%. The deviations are 10, −15, 5, 0 and 0; their squares sum to 350; divided by 4 that is 87.5; the square root is about 9.4%. The fund’s returns typically varied by around 9 percentage points from the average.
Figures are for illustration only — not a forecast or a recommendation.
Related terms
Sharpe ratio
The return earned above a baseline, such as a Treasury bill yield, for each unit of volatility taken; higher means more return per unit of risk.
Beta
How much a fund or stock has tended to move relative to its benchmark; a beta of 1.2 suggests swings about 20% larger in either direction.
Alpha
The return a fund earned above or below what its benchmark or level of market risk would explain — a rough gauge of a manager’s value-add.
Diversification
Spreading money across different investments so that a loss in any one of them has a limited effect on the whole portfolio.
Asset allocation
How you divide money across asset classes such as equity, debt, gold and cash — the biggest single driver of a portfolio’s risk and behaviour.