Higher-return categories like mid-cap and small-cap equity also tend to swing more from year to year. Statisticians measure that swing using standard deviation. For example, if a category has historically delivered an average return of 18% with a standard deviation of 30%, it broadly means that in a typical year the return could land anywhere between roughly 18% − 30% = −12% and 18% + 30% = 48%. The higher the standard deviation, the wider this range — and the more your portfolio value can move up or down in any given year, even if the long-term average holds over time.

Standard deviation measures volatility — how much returns swing year to year — and is a useful way to compare how bumpy the ride might be. But it is not the only kind of investment risk. Liquidity risk, concentration risk, credit risk, and the depth of a downturn (not just how often swings happen) also matter, and are not captured by this one number.

Technical note: the range shown above reflects roughly one standard deviation, which — under a normal-distribution assumption — covers only about two-thirds of outcomes. Actual returns can and do fall outside this range, and real-world equity returns (particularly mid-cap and small-cap) are not perfectly symmetric: downside moves have historically been sharper and more frequent than a symmetric distribution would predict, so actual losses can exceed the lower bound shown above. This is a general illustration only, based on round numbers, and not a prediction, guarantee, or recommendation for any specific fund or category. Actual historical returns and volatility vary by scheme, category, and time period.