Abstract - Software code review is a well-established software quality practice. Recently, Modern Code Review (MCR) has been widely adopted in both open source and industrial projects. To evaluate the impact that characteristics of MCR practices have on software quality, this paper comparatively studies MCR practices in defective and clean source code files. We investigate defective files along two perspectives: 1) files that will eventually have defects (i.e., future-defective files) and 2) files that have historically been defective (i.e., risky files). Through an empirical study of 11,736 reviews of changes to 24,486 files from the Qt open source system, we find that both future-defective files and risky files tend to be reviewed less rigorously than their clean counterparts. We also find that the concerns addressed during the code reviews of both defective and clean files tend to enhance evolvability, i.e., ease future maintenance (like documentation), rather than focus on functional issues (like incorrect program logic). Our findings suggest that although functionality concerns are rarely addressed during code review, the rigour of the reviewing process that is applied to a source code file throughout a development cycle shares a link with its defect proneness.
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