Group versus Individual Review Requests: Tradeoffs in Speed and Quality at Mozilla Firefox

The speed at which code changes are integrated into the software codebase, also referred to as code review velocity, is a prevalent industry metric for improved throughput and developer satisfaction. While prior studies have explored factors influencing review velocity, the role of the review assignment process, particularly the `group review request', is unclear. In group review requests, available on platforms like Phabricator, GitHub, and Bitbucket, a code change is assigned to a reviewer group, allowing any member to review it, unlike individual review assignments to specific reviewers. Drawing parallels with shared task queues in Management Sciences, this study examines the effects of group versus individual review requests on velocity and quality. We investigate approximately 66,000 revisions in the Mozilla Firefox project, combining statistical modeling with practitioner views from a focus group discussion. Our study associates group reviews with improved review quality, characterized by fewer regressions, while having a negligible association with review velocity. Additional perceived benefits include balanced work distribution and training opportunities for new reviewers.