The percentage of pull requests that were merged without review.

Why it matters: Unreviewed pull requests can present a risk to your codebase. In the short term, the review process is often the final opportunity to catch mistakes before the pull request is merged. Over the long term, the review process ensures knowledge sharing across the engineering team.

How to use it: If leaving some pull requests out of the review process is intentional, take care to ensure that the pull requests left unreviewed are matching your expectations. Otherwise, you may want to adjust your review requirements. 

How it's calculated: If the author of a PR merges it without any comments or approvals, it's considered unreviewed.

Benchmark: The top quartile of engineering organizations have a 0% Unreviewed Pull Request rate, and many top performing teams automate the enforcement of the code review process to ensure all work gets reviewed.