Mean squared logarithmic error (MSLE) averages squared differences on the log1p scale.
Details
$$\mathrm{MSLE}=\frac{1}{n}\sum_{i=1}^n [\log(1+obs_i)-\log(1+pred_i)]^2.$$
MSLE is non-negative and zero is ideal. It emphasizes relative differences
and requires non-negative observations and predictions; otherwise it returns
NA with a warning. The log1p convention is a package choice. Missing-value
handling follows bias().
References
Hodson, T. O. (2022). Root mean square error (RMSE) or mean absolute error (MAE): When to use them or not. Geoscientific Model Development, 15, 5481-5487. doi:10.5194/gmd-15-5481-2022
