 These deviations are called "residuals" when the calculations are performed over the data sample that was used for estimation and are called "errors" (or prediction errors) when computed out-of-sample.The RMSD serves to aggregate the magnitudes of the errors in predictions for various times into a single measure of predictive power.RMSD is a measure of accuracy, to compare forecasting errors of different models for a particular dataset and not between datasets, as it is scale-dependent.