Measuring coupling and cohesion: an information-theory approach

The design of software is often depicted by graphs that show components and their relationships. For example, a structure chart shows the calling relationships among components. Object oriented design is based on various graphs as well. Such graphs are abstractions of the software, devised to depict certain design decisions. Coupling and cohesion are attributes that summarize the degree of interdependence or connectivity among subsystems and within subsystems, respectively. When used in conjunction with measures of other attributes, coupling and cohesion can contribute to an assessment or prediction of software quality. Let a graph be an abstraction of a software system and let a subgraph represent a module (subsystem). The paper proposes information theory based measures of coupling and cohesion of a modular system. These measures have the properties of system level coupling and cohesion defined by L.C. Briand et al. (1996; 1997). Coupling is based on relationships between modules. We also propose a similar measure for intramodule coupling based on an intramodule abstraction of the software, rather than intermodule, but intramodule coupling is calculated in the same way as intermodule coupling. We define cohesion in terms of intramodule coupling, normalized to between zero and one. We illustrate the measures with example graphs. Preliminary analysis showed that the information theory approach has finer discrimination than counting.