Accuracy & evaluation

sMAPE

Also called symmetric MAPE

sMAPE is a percentage error that divides by the average of |actual| and |forecast|, so zeros on one side do not explode as violently as MAPE. It is still a percent, and it still is not a volume-weighted score.

There are two common formulas. They do not agree. If you report sMAPE, paste the formula in the footnote. Future-you will thank you during a vendor bake-off.

sMAPE treats over- and under-forecasts more symmetrically than MAPE, which is why it showed up in forecasting competitions. For inventory, WAPE or a custom cost still maps better to pallets.