The time between when the first commit is authored to when a pull request is merged.

Why it matters: PR Cycle Time represents your team’s time-to-market, or how quickly software is delivered to customers. Low PR Cycle Time often equates to higher output and more efficient teams. It is also correlated with higher stability, giving your team the ability to quickly respond to issues with change.

 This is a success metric that you can use across individuals, teams, or cohorts to make certain that every process modification is improving engineering speed.

How to use it: You can treat PR Cycle Time as your speedometer. Use it to understand baseline productivity, and check any major change in processes against this to make sure it had a positive (or perhaps non-negative) effect on productivity. 

This metric is not diagnostic, so to identify why PR Cycle Time is low, you’ll want to look at metrics that make up smaller components of the software development process, such as Time to Open, Time to First Review, and Time to Merge.

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Note: PR Cycle Time starts at the first commit on the branch -- and not (necessarily) the first commit once the branch is opened.