busfactor

Bus factor, also known as the truck factor, is a project risk metric that estimates the minimum number of team members who must be removed from a project (for example due to illness or departure) before the project’s progress stalls or fails. It serves as a heuristic for the resilience of a project to the loss of key personnel.

The concept originates in software engineering and startup culture and characterizes the concentration of critical knowledge and decisions in a small number of individuals. A low bus factor suggests that a few people hold most of the essential know-how, while a high bus factor indicates broader distribution of knowledge and responsibilities.

Measurement of the bus factor is not an exact calculation; it is a rough assessment. It involves identifying who knows crucial aspects of the codebase, architecture, dependencies, build and release processes, and operational runbooks, and evaluating how dependent the project is on those individuals. Factors such as documentation, tests, onboarding, and code ownership influence the factor.

Mitigation strategies to increase the bus factor include thorough documentation, shared ownership of code and processes, pair programming, code reviews, automated tests, continuous integration and deployment, regular knowledge-sharing sessions, and clear runbooks. Strong onboarding and cross-training programs also help distribute critical knowledge more evenly.

Critics note that the bus factor is a simplistic measure that can mislead if taken too literally. It does not capture all risks, may encourage gaming the metric, and should be considered alongside other indicators of project health and resilience.