Calculating the "truck factor" for GitHub projects

The idea of a truck or bus factor (or number) has been-morbidly, perhaps-bandied about in development projects for many years. It is a rough measure of how many developers would have to be lost (e.g. hit by a bus) to effectively halt the project. A new paper [PDF] outlines a method to try to calculate this number for various GitHub projects. Naturally, it has its own GitHub project with a description of the methodology used and some of the results. It was found that 46% of the projects looked at had a truck factor of 1, while 28% were at 2. Linux scored the second highest at 90, while the Mac OS X Homebrew package manager had the highest truck factor at 159.