| Abstract | The main goal of the review presented in this paper is to examine when to use expert judgment, when to use formal models, and, when to combine these two approaches when estimating software development work effort. Sixteen relevant studies were identified and reviewed. The review found that the average accuracy of expert judgment-based effort estimates was better than the average accuracy of the models in ten of the sixteen studies. Two indicators of higher accuracy of judgment-based effort estimates seem to be that: i) the estimation models are not calibrated to the organization using the model, and, ii) the experts possess important context information not included in the formal estimation models. The use of models, on the other hand, may be particularly useful in estimation situations believed to lead to stronger than usual degree of over-optimism, particularly very large projects. Five of the reviewed studies evaluated estimates based on a combination of expert judgment and model. In all five studies the average accuracy of combination-based effort estimates had similar or more accurate estimates than the average accuracy of the expert estimates and of the best model. |
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