1 Introduction

Class diagram quality is clearly a crucial issue that must be evaluated (and improved if necessary) in order to get quality OO software. This fact is corroborated by the huge amount of metrics that can be applied to UML (OMG, 2003c) class diagrams at a high level design stage, that have been recorded in relevant literature. Most of these studies are focused on the measurement of internal quality attributes such as structural complexity, coupling, size, etc. However, none of the proposed metrics take into account the added complexity involved when class diagrams are complemented by expressions written in Object Constraint Language (OCL).

OCL, defined by OMG (2003b), has became a fundamental language in developing OO software using UML, as it allows complete and consistent UML modelling. A model specified in a combination of the UML and OCL languages is mentioned in Warmer and Kleppe (2003), as a UML/OCL combined model, or just a UML/OCL model. OCL enriches, for example, UML class diagrams with expressions that specify semantic properties of a model (Gogolla and Richters, 2001). The OCL expressions are unambiguous and make the model more precise and more detailed (Warmer and Kleppe, 2003) improving its understandability at early stages...