Measure contributor activity levels from commits, PRs and reviews over time.
Active contributor ratio measures how much of a project's historical contributor base is still engaged. Formula: active_ratio = contributors_with_commits_in_period / total_contributors. Pairing this with the count of 'core' contributors (a common threshold is >10 commits in the period) surfaces bus-factor risk — how much the project depends on a small handful of people.
active_ratio_percent = (active_contributors / total_contributors) × 100
For long-lived open source projects, 10-25% active-in-period is common and healthy since most historical contributors made one-off fixes. For internal product teams, you'd expect this closer to 80-100% since most contributors are current employees.
Bus factor is the minimum number of people who could leave before a project stalls. A low core-contributor count (1-2 people) means critical knowledge is concentrated and creates high organizational risk.
Use `git shortlog -sn --since=<date>` for the active list and `git shortlog -sn` (no date filter) for all-time contributors, then compare unique author counts.
Not necessarily — a healthy funnel of new one-time contributors (e.g. drive-by PRs) is a good sign for open source growth, even if the active ratio looks low, as long as core maintainers remain stable.