Investigating the Use of Duration-Based Moving Windows to Improve Software Effort Prediction

To date most research in software effort estimation has not taken into account any form of chronological split when selecting projects for training and testing sets. A chronological split represents the use of a project's starting and completion dates, such that any model that estimates effort for a new project p only uses as its training set projects that were completed prior to p's starting date. Three recent studies investigated the use of chronological splits, using a type of chronological split called a moving window, which represented a subset of the most recent projects completed prior to a project p's starting date. They found some evidence in favour of using windows whenever projects were recent. These studies all defined window sizes as being fixed numbers of recent projects. In practice, we suggest that estimators are more likely to think in terms of elapsed time than the size of the data set, when deciding which projects to include in a training set. Therefore, this paper investigates the effect on accuracy when using moving windows of various durations to form training sets on which to base effort estimates. Our results show that the use of windows based on duration can affect the accuracy of estimates (in this data set, a window of about three years duration appears best), but to a lesser extent than windows based on a fixed number of projects.