The Theory of Relative Dependency: Higher Coupling Concentration in Smaller Modules

Recent studies have repeatedly found that smaller modules are proportionally more defect-prone. In this article, the authors formulate and test a hypothesis stating that smaller modules are proportionally more coupled, given that dependencies caused by coupling have been consistently associated with defect-proneness. Strong evidence supports this hypothesis. Furthermore, refactoring exacerbates this effect. On the basis of this study's highly consistent results, the authors state the empirically based theory of relative dependency. That is, in large-scale software systems, smaller modules will be proportionally more dependent compared to larger ones. These findings have two implications for practice. First, we now have an empirically supported mechanism explaining the observations that defect concentration is higher in smaller modules. Practitioners can use this mechanism as evidence while seeking resources and support to revise or amend their organizations' quality assurance and quality control practices. Second, particularly for the projects that refactor extensively, such as those using agile methods, focusing defect detection activities on smaller modules will increase their efficiency and effectiveness even more.