Chapter

Principles of Forecasting

Volume 30 of the series International Series in Operations Research & Management Science pp 417-439

Combining Forecasts

  • J. Scott ArmstrongAffiliated withThe Wharton School, University of Pennsylvania

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Abstract

To improve forecasting accuracy, combine forecasts derived from methods that differ substantially and draw from different sources of information. When feasible, use five or more methods. Use formal procedures to combine forecasts: An equal-weights rule offers a reasonable starting point, and a trimmed mean is desirable if you combine forecasts resulting from five or more methods. Use different weights if you have good domain knowledge or information on which method should be most accurate. Combining forecasts is especially useful when you are uncertain about the situation, uncertain about which method is most accurate, and when you want to avoid large errors. Compared with errors of the typical individual forecast, combining reduces errors. In 30 empirical comparisons, the reduction in ex ante errors for equally weighted combined forecasts averaged about 12.5% and ranged from 3 to 24%. Under ideal conditions, combined forecasts were sometimes more accurate than their most accurate components.

Keywords

Consensus domain knowledge earnings forecasts equal weights group discussion uncertainty