Standard Deviation Explained
What is it?
Standard deviation measures how much a fund's returns fluctuate around its own average return — a higher standard deviation means returns swing more widely from year to year (or month to month), while a lower standard deviation means returns are more steady and predictable.
Why should you care?
Two funds can have the exact same average return over 5 years while one got there through a smooth, steady path and the other through wild swings. Standard deviation captures that difference, which matters a lot for how comfortable the ride feels — and for whether you're likely to panic-sell during a rough patch.
Real-life example
Fund A and Fund B both average 12% annual returns over 5 years. Fund A's yearly returns were roughly 10%, 13%, 11%, 14%, 12% — a low standard deviation, steady path. Fund B's yearly returns were roughly -5%, 25%, 8%, 22%, 10% — the same average, but a much higher standard deviation, meaning a much bumpier ride to the same destination.
Common mistakes
- Comparing two funds purely on average return without checking standard deviation.
- Choosing a high standard deviation fund without honestly assessing whether you can tolerate its swings without panic-selling.
- Treating standard deviation as inherently bad — some investors with long horizons and high risk tolerance may accept it for potentially higher returns.
Same average return, different standard deviation (illustrative)
| Fund | Yearly returns pattern | Average | Standard deviation |
|---|---|---|---|
| Fund A | 10%, 13%, 11%, 14%, 12% | 12% | Low |
| Fund B | -5%, 25%, 8%, 22%, 10% | 12% | High |
FAQ
Is a lower standard deviation always preferable?
Not universally — it depends on your goal's time horizon and your own tolerance for volatility. A long-horizon investor may knowingly accept higher standard deviation for potentially higher long-term growth.
How is standard deviation different from beta?
Standard deviation measures a fund's own volatility in isolation; beta measures its volatility specifically relative to a benchmark or the market (see Beta Explained).
Does standard deviation predict future risk perfectly?
It's based on historical returns, so it's a useful guide but not a guarantee — future volatility can differ from the past, especially during unprecedented market conditions.
How does standard deviation feed into Sharpe and Sortino ratios?
Both ratios use volatility measures (standard deviation, or its downside-only variant) in their denominator to express return per unit of risk taken — see Sharpe Ratio vs Sortino Ratio.
See this concept applied to your own portfolio
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