Questioning software maintenance metrics: A comparative case study

Context: Many metrics are used in software engineering research as surrogates for maintainability of software systems. Aim: Our aim was to investigate whether such metrics are consistent among themselves and the extent to which they predict maintenance effort at the entire system level. Method: The Maintainability Index, a set of structural measures, two code smells (Feature Envy and God Class) and size were applied to a set of four functionally equivalent systems. The metrics were compared with each other and with the outcome of a study in which six developers were hired to perform three maintenance tasks on the same systems. Results: The metrics were not mutually consistent. Only system size and low cohesion were strongly associated with increased maintenance effort. Conclusion: Apart from size, surrogate maintainability measures may not reflect future maintenance effort. Surrogates need to be evaluated in the contexts for which they will be used. While traditional metrics are used to identify problematic areas in the code, the improvements of the worst areas may, inadvertently, lead to more problems for the entire system. Our results suggest that local improvements should be accompanied by an evaluation at the system level.