Experts often add domain knowledge to model-based forecasts while aiming to reduce forecast errors. Indeed, there is some empirical evidence that expert-adjusted forecasts improve forecast quality. However, surprisingly little is known about what experts actually do. Based on a large and detailed database concerning monthly pharmaceutical sales forecasts, we examine whether expert adjustment is predictable. We find substantial evidence that the size, the relative size and even the sign of such adjustment show positive-valued dynamics. The main drivers of current expert adjustments are past adjustment and past model-based forecast errors. Our findings are also that experts' adjustment may suffer from double counting and that trust in the statistical model is not large. An implication is that models may need improvement. Also, experts need to focus on other variables than past sales data when adjusting model-based forecasts. Finally, the method to evaluate the quality of experts' adjustment needs to be modified.

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Econometric Institute Research Papers
Report / Econometric Institute, Erasmus University Rotterdam
Erasmus School of Economics

Franses, P. H., & Legerstee, R. (2007). Dynamics of expert adjustment to model-based forecast (No. EI 2007-35). Report / Econometric Institute, Erasmus University Rotterdam. Retrieved from