Experiments with Coupling and Cohesion Metrics in a Large System

We apply coupling and cohesion metrics to various decompositions of a large telecommunications system. Several findings emerge: 1) There is a baseline level of coupling and cohesion that represents the average connectedness of any pair of files in the system. 2) As would be expected, measures of cohesion are greater than this baseline in well-defined subsystems. 3) Interestingly, and contrary to intuition, measures of coupling for such subsystems tend also to be greater than the baseline and to increase as cohesion increases. 4) It is possible to easily calculate upper bounds for coupling and cohesion that no subsystem in a given system can exceed. 5) Measuring quality by subtracting coupling from cohesion, as has been proposed in the literature, gives anomalous results since coupling and cohesion are frequently not on the same scale. We propose improved coupling, cohesion and quality metrics that normalize for the baseline and ceiling levels in a given system. Using our proposed metrics it should be possible to compare different systems, something that the current metrics do not permit.