Statisticscalendar_todayLast updated: Apr 2026
What is Standard Deviation?
/ˈstændəd ˌdiːvɪˈeɪʃən/
Standard deviation measures how spread out a set of values is around its average (mean). A low standard deviation means values cluster close to the mean; a high one means they are spread widely.
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Everyday Example
If your test scores over a term are 70, 71, 69, 70, 72, the standard deviation is tiny — consistent performance. Scores of 40, 95, 55, 90, 45 have a high standard deviation — wildly inconsistent.
publicReal-World Application
“Investment fund managers are evaluated not just on returns but on risk-adjusted returns — a fund returning 20% with very high standard deviation (wild swings) is riskier than one returning 15% consistently.”
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Did you know?
Standard deviation was developed by statistician Karl Pearson in 1893. The formula appears simple but carries enormous analytical power across sciences, finance, and quality control.
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Key Insight
The "68-95-99.7 rule": in a normal distribution, 68% of values fall within 1 standard deviation of the mean, 95% within 2, and 99.7% within 3. Most real-world data follows this pattern.
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