Mean absolute percentage error (MAPE) averages absolute error relative to each observation.
Details
$$\mathrm{MAPE}=\frac{100}{n}\sum_{i=1}^n \left|\frac{obs_i-pred_i}{obs_i}\right|.$$
MAPE is a non-negative percentage; zero is ideal. It returns NA with a
warning when any observation is zero and can
disproportionately weight errors near zero. Missing-value handling follows
bias().
References
Hyndman and Koehler (2006). See mdae().
