Standard error of prediction (SEP) is the sample standard deviation of prediction errors after removing their mean error (ME).
Details
$$\mathrm{SEP}=\sqrt{\frac{1}{n-1}\sum_{i=1}^{n} \left[(obs_i-pred_i)-\frac{1}{n}\sum_{j=1}^{n}(obs_j-pred_j)\right]^2}.$$
SEP has response units, is non-negative, and zero is ideal. Unlike RMSE, it
removes constant bias. It returns NA with a warning when fewer than two
retained pairs remain. Missing-value handling follows bias().
References
Bellon-Maurel et al. (2010). See rpd().
