High Quality Content by WIKIPEDIA articles! The root mean square deviation (RMSD) or root mean square error (RMSE) is a frequently-used measure of the differences between values predicted by a model or an estimator and the values actually observed from the thing being modeled or estimated. RMSD is a good measure of precision. These individual differences are also called residuals, and the RMSD serves to aggregate them into a single measure of predictive power. For an unbiased estimator, the RMSE is the square root of the variance, known as the standard error.