A number of functional size measurement (FSM) methods have been proposed in the literature, but so far there has been little systematic evaluation of these methods. This paper describes a controlled experiment which compares function point analysis, a standard FSM method supported by the International Function Point Users Group (IFPUG FPA) and OO-method function points (OOmFP), a recently proposed FSM method for sizing object-oriented software systems that are developed using the OO-Method approach. The goal is to investigate whether OOmFP results in better size assessments within the context of an OO-Method development process. The methods are compared using two criteria defined in the ISO 14143-3: reproducibility and accuracy. The results show that, within its context, OOmFP is more consistent and accurate than IFPUG FPA.