65% of popular GitHub projects have bus factor ≤ 2

Avelino, Passos, Hora and Valente 2016 analyzed 133 popular GitHub projects and found that 65% had a truck factor (bus factor) ≤ 2 — meaning that two contributor departures would leave the project effectively unmaintained.

The "popular GitHub project" sample is not a worst case; it is a sampled distribution of projects with active community engagement and substantial codebase scale. The bus-factor concentration measures contributor structure, not project quality. The same concentration applies, with stronger force, in academic research where authorship structures are typically smaller and contributor turnover (C-0017) is structurally guaranteed.

This is one of the few systematically-measured studies of operational concentration in software projects, and it generalizes by analogy to research data: the data underlying most published papers is maintained by a small number of people, and the loss of any one of them is a high-probability preservation event.