= Sample standard deviation
{title2=$S=\sqrt{\frac1{n-1}\sum_i(X_i-\overline X)^2}$}
The sample standard deviation is the nonnegative square root of the usual <sample variance> for $n\ge2$. The displayed convention uses divisor $n-1$. Although the <sample variance> is an <unbiased estimator> of the population <variance> under independent identically distributed sampling with finite <variance>, taking its square root does not in general give an <unbiased estimator> of the population <standard deviation>.
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