The MSE of an estimator, $\widehat{\theta },$ is defined as E[($\widehat{\theta }$ − θ‎)²] where θ‎ is the estimated parameter. The MSE is equal to the sum of the variance of the estimator and the square of its bias. MSE is often used as a criterion for comparing alternative estimators of the same parameter: the one with smaller MSE is preferred.