bus factor
expressionops cultureprogramming
The number of people who would have to be hit by a bus before a project stalls for lack of knowledge.
A bus factor of one means a single person holds critical, undocumented knowledge - a serious risk. Teams raise it deliberately through documentation, pairing, and code review so that no component depends on exactly one irreplaceable head.
The bus factor is the number of people who would have to be hit by a bus before a project is in serious trouble, and the morbid framing is the point: it forces an uncomfortable question that politer language lets people avoid. A bus factor of one means a single person's absence stops the work.
What makes it useful is that it measures something organizations systematically fail to see. The person who is the bus factor is usually productive, reliable and trusted, so nothing looks wrong. The risk is invisible precisely because the system is working, and it only materializes when they take leave, change teams, or resign, at which point the knowledge turns out to have been in one head and nowhere else.
Raising it is unglamorous and cheap compared to the alternative. Documentation that describes why rather than what, pair work on the systems only one person touches, rotating who handles which incident, and deliberately sending the expert on holiday to find out what breaks. The last one is the honest test: a team that cannot function while someone is away has already discovered its bus factor, it just has not admitted it.
Also known as: truck factor, lottery factor, bus number