Software systems must change to adapt to new functional requirements and new nonfunctional requirements. This is called software revision. However, not all the modules within the system need to be changed during each revision. In this paper, we study how frequently each module is modified. Our study is performed through comparing the stability of peer software modules. The study is performed on six open-source Java projects: Ant, Flow4j, Jena, Lucence, Struct, and Xalan, in which classes are identified as basic software modules. Our study shows (1) about half of the total classes never changed; (2) frequent changes occur to small number of classes; and (3) the number of changed classes between current release and next release has no significant relations with the time duration between current release and next release. Keywords: software evolution; software revision; software stability; class stability; open-source project; Java clas
