How it's calculated
s = √( Σ(xᵢ − x̄)² ÷ (n − 1) )
xᵢ = each value, x̄ = mean of the list, n = count, Σ = sum over all values. Shown is the sample standard deviation (n − 1); the population version divides by n.
Common questions
Should I use sample or population SD?
Use sample SD (n-1) when your data is a sample of a larger group. Use population SD (n) when you have the entire population.
How does standard deviation relate to variance?
Standard deviation is the square root of the variance, which puts the spread back into the original units.