  Truck Factor (TF) is a metric proposed by the agile community as a tool to
identify concentration of knowledge in software development environments. It
states the minimal number of developers that have to be hit by a truck (or
quit) before a project is incapacitated. In other words, TF helps to measure
how prepared is a project to deal with developer turnover. Despite its clear
relevance, few studies explore this metric. Altogether there is no consensus
about how to calculate it, and no supporting evidence backing estimates for
systems in the wild. To mitigate both issues, we propose a novel (and
automated) approach for estimating TF-values, which we execute against a corpus
of 133 popular project in GitHub. We later survey developers as a means to
assess the reliability of our results. Among others, we find that the majority
of our target systems (65%) have TF <= 2. Surveying developers from 67 target
systems provides confidence towards our estimates; in 84% of the valid answers
we collect, developers agree or partially agree that the TF's authors are the
main authors of their systems; in 53% we receive a positive or partially
positive answer regarding our estimated truck factors.
