1. Introduction
Effort and duration (hereafter termed cost) estimates are important deliverables at the point of contracting a software project. The client requires the cost estimates to make an investment decision (i.e. investing in a software project) and to budget the investment. The project manager uses cost estimates to plan the project, e.g. staffing. Unfortunately, software projects still run over budget and time. As a consequence, much research effort in software engineering has been devoted to this problem over the last 25 years. A plethora of prediction models have been proposed and evaluated to improve prediction accuracy.