You don't, and it's foolish to try. Open source is philanthropic and high-trust. Start putting numbers and formulae on top of that and watch the culture crumble

You don't, and it's foolish to try. Open source is philanthropic and high-trust. Start putting numbers and formulae on top of that and watch the culture crumble as people optimize for newly-established metrics. In fact, didn't that happen recently with Hacktoberfest where the people with the most contributions got T-shirts, and promptly people started gaming the metric just to win the T-shirt?