Function Point Analysis for Software Maintenance

Context: Software maintenance is required to fix defects, adapt to changes in the environment, and meet new or changed user requirements. The effort of these tasks need to be estimated to track progress, manage resources, and make decisions. Most widely used cost models use source lines of code (SLOC) as the software size input measure, due to its quantifiability and high correlation with effort. Estimating the SLOC of a project is very difficult in early stages of the software lifecycle. Function Points (FPs) represents software size by functions or modifications to functions, making them easier to calculate early in the lifecycle for new development projects or maintenance tasks. Several cost estimators use FPs to estimate the SLOC of a project to take advantage of existing cost models. Goal: Through empirical analysis, the authors want to determine whether FPs can effectively estimate maintenance tasks, as a better alternative to using SLOC as a software size metric. Additionally, the authors will demonstrate that FPs to SLOC ratios add uncertainty to effort estimates. Method: The empirical analysis will be run on Unified Code Count (UCC)'s dataset, a software tool maintained by University of Southern California (USC). Results: The analyses found that separating projects adding new functions from those modifying existing functions resulted in improved estimation models using FPs. The effort estimation model for projects adding functions to UCC had high prediction accuracy statistics, but less impressive results for projects modifying existing functions in UCC. The effort estimation accuracy became unsatisfactorily low when using a FPs to SLOC ratio. Conclusions: Cost estimators should not use FPs to SLOC ratios for effort estimation due to low prediction accuracy. FPs is only an effective size measure for a portion of UCC's maintenance tasks - specifically for the projects adding new functions to UCC. Another size measure may need to be considered that might be more effective independently or in conjunction with FPs for all of UCC's maintenance tasks.