The role of the measure of functional complexity in effort estimation

Background. Currently there are several definitions of measures that should represent the size of software functional requirements. These measures have gained a quite relevant role, since they are one of the few basis upon which effort estimation can be based. However, traditional Functional Size Measures do not take into account the amount and complexity of the elaboration required, concentrating instead on the amount of data accessed or moved. This is a problem, when it comes to effort estimation, since the amount and complexity of the required data elaborations affect the implementation effort, but are not adequately represented by the current measures (including the standardized ones). Objective. The paper evaluates different types of functional size measures as effort estimators. Moreover, the consequences of taking into consideration also the amount and complexity of required elaboration in the effort estimation models are evaluated. Methods. In this paper we take into consideration a representative set of functional size measures (namely, function points, COSMIC function points and use case points) and a recently proposed elaboration complexity measure (Paths) and evaluate how well these measures are correlated with the development effort. To this end, we measured a set of 17 projects and analyzed the resulting data. Results. We found that it is possible to build statistically valid models of the development effort that use the functional size and complexity measures as independent variables. In fact, we discovered that using the measure of elaboration complexity in addition to the functional size substantially improves the precision of the fitting. Conclusions. The analysis reported here suggests that a measure of the amount and complexity of elaboration required from a software system should be used, in conjunction with traditional functional size measures, in the estimation of software development effort. Further investigations, involving a greater number of projects, are however needed to confirm these findings