The RMSE of an an estimator of a parameter is the square-root of the mean squared error (MSE) of the estimator. In symbols, if X is an estimator of the parameter t, then

RMSE(X) = ( E( (X−t)² ) )^½.

The RMSE of an estimator is a measure of the expected error of the estimator. The units of RMSE are the same as the units of the estimator. See also mean squared error.