164. MD

164.1. MD (Mean Deviation / Bias)

Average of the signed forecast errors . Captures bias — whether the forecast systematically over- or under-shoots.

• : forecast is unbiased on average
• : forecast under-predicts (actuals exceed forecast)
• : forecast over-predicts

Caveat. Positive and negative errors cancel out, so MD says nothing about accuracy — a forecast that alternates wildly between and has MD . Pair MD with MAE / RMSE to separate bias from accuracy.