Forecasting of software development work effort: Evidence on expert judgement and formal models

The review presented in this paper examines the evidence on the use of expert judgement, formal models, and a combination of these two approaches when estimating (forecasting) software development work effort. Sixteen relevant studies were identified and reviewed. The review found that the average accuracy of expert judgement-based effort estimates was higher than the average accuracy of the models in ten of the sixteen studies. Two indicators of higher accuracy of judgement-based effort estimates were estimation models not calibrated to the organization using the model, and important contextual information possessed by the experts not included in the formal estimation models. Four of the reviewed studies evaluated effort estimates based on a combination of expert judgement and models. The mean estimation accuracy of the combination-based methods was similar to the best of that of the other estimation methods.