Relative root mean squared error (RRMSE) expresses RMSE as a percentage of the absolute observed mean.
Details
$$\mathrm{RRMSE}=100\, \frac{\sqrt{n^{-1}\sum_{i=1}^{n}(obs_i-pred_i)^2}}{|\bar{obs}|}.$$
RRMSE is non-negative and zero is ideal. It returns NA with a warning for
a zero observed mean and is unstable when that mean is near zero. This mean-normalized convention
differs from the standard-deviation normalization in nrmse(). Missing-value
handling follows bias().
References
Willmott, C. J., Ackleson, S. G., Davis, R. E., Feddema, J. J., Klink, K. M., Legates, D. R., O'Donnell, J., and Rowe, C. M. (1985). Statistics for the evaluation and comparison of models. Journal of Geophysical Research, 90, 8995-9005. doi:10.1029/JC090iC05p08995
