Formula
σ² = Σ(xᵢ − μ)² / N
s² = Σ(xᵢ − x̄)² / (n−1)
σ² = population variance
s² = sample variance
μ / x̄ = mean
N = population size; n = sample size
Pro Tips
Use sample variance (s²) when your data is a sample from a larger population — the n−1 denominator corrects for bias.
Standard deviation is simply the square root of variance — both measure spread, but SD is in the original units.