Are 20% of Classes Responsible for 80% of Refactorings?

The 80-20 rule is well-known in the real-world. When applied to bugs, it suggests that 80% of bugs arise in just 20% of classes. One research question that has yet to be explored is whether the same rule applies to refactoring activity. In other words, do 20% of classes account for 80% of refactorings applied to a system? In this short paper, we explore this question using data from seven open-source systems drawn from two previous studies. In each case, we explore whether the 80-20 rule applies and suggest why. Results showed limited evidence of an 80-20 rule; in the two systems where it was evident, the refactoring profile implied firstly, a large-scale movement of class fields and methods and, secondly, the deliberate aim of collapsing the class hierarchy using inheritance-based refactorings.