168. MPE

168.1. MPE (Mean Percent Error)

Signed-percentage version of MD. Captures relative bias — whether the forecast over- or under-shoots in percentage terms.

• : forecast is unbiased on a percentage scale
• : under-prediction (actuals exceed forecast)
• : over-prediction

Caveats.

• Positive and negative errors cancel — same warning as MD. Pair with MAPE to separate bias from accuracy.
• Undefined / unstable when any or is small relative to the error.