# Population Variance $ \sigma^2 = \frac{\sum(x_i - \mu)^2}{N} $ - $N$ is the population size - $\mu$ is the population mean Variance measures the dispersion of a random variable and is the [[Standard Deviation]] squared. For a Bernoulli trial this simplifies to: $ \sigma^2 = p \times (1-p) $ - $p$ is the probability of success of the Bernoulli trial Use [Sample Variance](Sample%20Variance.md) if your data is from a sample only.