What is Standard Deviation? (Plain English Definition)
Definition: Standard deviation measures how much an investment's returns vary from its average return, quantifying the volatility or risk of the investment.
Standard Deviation Explained Simply
Standard deviation is a statistical measure that quantifies how much an investment's returns fluctuate around its average. A higher standard deviation means returns are more spread out and volatile. A lower standard deviation means returns are more consistent and predictable.
For stocks, the S&P 500 has a historical standard deviation of about 15-16% per year. This means that in any given year, the return is likely to fall within one standard deviation of the average roughly 68% of the time. If the average return is 10% and the standard deviation is 15%, you can expect returns between -5% and +25% in about two-thirds of all years.
Standard deviation is the most common measure of investment risk used in finance. It is a key input for calculating the Sharpe ratio and other risk-adjusted return measures. Bond ETFs typically have standard deviations of 3-8%, while equity ETFs range from 12-25% depending on the type of stocks held. Understanding standard deviation helps you compare the riskiness of different ETFs on a quantitative basis.
Standard Deviation Example
ETF A has an average annual return of 10% and a standard deviation of 12%. ETF B also averages 10% per year but has a standard deviation of 25%. In any given year, ETF A's return will likely fall between -2% and +22% about two-thirds of the time. ETF B could range from -15% to +35%. Both have the same average return, but ETF B gives you a much bumpier ride. Most investors would prefer ETF A's smoother returns.
Why Standard Deviation Matters for ETF Investors
Standard deviation helps ETF investors quantify risk instead of relying on vague notions of safety or danger. When you see that a technology ETF has a standard deviation of 22% versus a bond ETF at 5%, you have concrete numbers to compare their volatility. For ETF investors, checking an ETF's standard deviation helps set expectations for how much your portfolio might fluctuate. If a fund has an 18% standard deviation, you should be prepared for declines of 18% or more in any given year -- it is not unusual, it is expected. Understanding this helps you choose ETFs with volatility levels you can tolerate and avoid panic-selling during normal market fluctuations.
Standard Deviation vs Volatility
| Standard Deviation | Volatility |
|---|---|
| Standard deviation measures how much an investment's returns vary from its average return, quantifying the volatility or risk of the investment. | See full definition of Volatility |
While standard deviation and volatility are related concepts, they serve different purposes in the world of ETF investing. Understanding both terms helps you make more informed decisions about which funds to include in your portfolio and how to evaluate their performance.
Related Terms
Deepen your understanding of ETF investing by exploring these related concepts:
Volatility
Volatility measures how much and how quickly an investment's price changes, with higher volatility meaning larger and more frequent price swings.
Beta
Beta measures how much an investment's price tends to move relative to the overall market, indicating its volatility compared to a benchmark.
Sharpe Ratio
The Sharpe ratio measures an investment's risk-adjusted return by dividing its excess return above the risk-free rate by its standard deviation.
Risk-Adjusted Return
Risk-adjusted return measures an investment's return relative to the amount of risk taken, showing whether higher returns adequately compensate for higher risk.
Risk Tolerance
Risk tolerance is the degree of variability in investment returns that an investor is willing and able to withstand.
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