Object-oriented function points: An empirical validation

We present an empirical validation of object-oriented size estimation models. In previous work we proposed object oriented function points (OOFP), an adaptation of the function points approach to object-oriented systems. In a small pilot study, we used the OOFP method to estimate lines of code (LOC). In this paper we extend the empirical validation of OOFP substantially, using a larger data set and comparing OOFP with alternative predictors of LOC. The aim of the paper is to gain an understanding of which factors contribute to accurate size prediction for OO software, and to position OOFP within that knowledge. A cross validation approach was adopted to build and evaluate linear models where the independent variable was either a traditional OO entity (classes, methods, association, inheritance, or a combination of them) or an OOFP-related measure. Using the full OOFP process, the best size predictor achieved a normalized mean squared error of 38%. By removing function point weighting tables from the OOFP process, and carefully analyzing collected data points and developer practices, we identified several factors that influence size estimation. Our empirical evidence demonstrates that by controlling these factors size estimates could be substantially improved, decreasing the normalized mean squared error to 15%-in relative terms, a 56% reduction.