Relative absolute error (RAE) compares total absolute error with total absolute error from predicting the observed mean.
Details
$$\mathrm{RAE}=\frac{\sum_{i=1}^n|obs_i-pred_i|} {\sum_{i=1}^n|obs_i-\bar{obs}|}.$$
RAE is non-negative and zero is ideal. One equals the observed-mean
absolute-error benchmark; values above one are worse. It returns NA with a
warning for constant observations, whose benchmark denominator is zero.
Missing-value handling follows bias().
References
Hyndman and Koehler (2006). See mdae().
