Coupling and strength, a la Harlan D. Mills

Building on the foundation championed by Harlan D. Mills, analysts and programmers can appraise the coupling and strength (cohesion) of the components of software systems, here termed "modules." Consistent with the stance of Harlan D. Mills, this paper shows the use of message tables for representing the designed or actual functionality of and data flow among the modules. From that base, this paper restates the definitions of coupling and strength to enable a more objective and practical appraisal of coupling and strength than has characterized the traditional approaches. This more objective and practical approach uses two decision tables, one for appraising the coupling and one for appraising the strength of the modules. To assist in appraising the strength of modules, this paper identifies four kinds of functions that are normally ignored in doing the strength appraisal. This paper includes simple worked-out examples, notes that the approach has been used in the field successfully from design time through testing, and includes some discussion of the applicability of this process of making better measurements of module coupling and strength.