Coupling metrics for object-oriented design

We describe and evaluate some recently innovated coupling metrics for object-oriented (OO) design. The Coupling Between Objects (CBO) metric of Chidamber and Kemerer (1991) is evaluated empirically using five OO systems, and compared with an alternative OO design metric called SAS, which measures the number of associations between a class and its peers. The NAS metric is directly collectible from design documents such as the Object Model of OMT. Results from all systems studied indicate a strong relationship between CBO and NAS, suggesting that they are not orthogonal. We hypothesised that coupling would be related to understandability, the number of errors and error density. So relationships were found for any of the systems between class understandability and coupling. Only limited evidence was found to support our hypothesis linking increased coupling to increased error density. The work described in this paper is part of the 'Metrics for OO Programming Systems' (MOOPS) project, which aims to evaluate existing OO metrics, and to innovate and evaluate new OO analysis and design metrics, aimed specifically at the early stages of development.