Standard deviation

Definition

Standard deviation in plain English

Standard deviation is a single number that tells you how spread out a set of values is — whether they're all clustered tightly together or scattered all over the place. Picture two classes with the same average test score of 80: in one everyone scored near 80, in the other half scored 50 and half scored 100. Same average, but the second class is far more spread out, and standard deviation captures exactly that. Technically it's the square root of the variance, and the square-root step matters because it puts the answer back into the original units (like actual points), so a standard deviation of 10 points means values typically sit about 10 points away from the average.

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