Bus Factor

A measure of key-person risk — the number of people who would have to be lost before a project or team stalls because critical knowledge lived only with them.

Bus factor (also called the 'truck factor' or 'lottery factor') is an informal measure of concentration risk on a team: the minimum number of people who would have to suddenly become unavailable — the morbid framing is 'hit by a bus,' though winning the lottery works just as well — before a project grinds to a halt because essential knowledge, access, or capability lived only in their heads. A bus factor of one is the danger zone: a single person is the sole holder of some critical piece, and their departure, illness, or extended absence would leave the team unable to function until someone painfully reconstructs what they knew.

The concept originated in software engineering, where it's easy for one developer to become the only person who understands a critical system, holds the credentials to a key service, or knows why a fragile piece of code works the way it does. But the risk generalizes to any knowledge work: the one salesperson who holds all the key client relationships, the single operations person who knows the undocumented month-end close process, the manager who is the only one with context on a strategic commitment. A low bus factor is a silent organizational vulnerability — everything runs fine right up until the key person is gone, at which point the cost of the concentration becomes suddenly and expensively visible.

Raising a team's bus factor is a deliberate practice, not something that happens on its own — because the natural gravity of work pulls the other way, toward whoever already knows something being the fastest person to do it again. The countermeasures are documentation (writing down the undocumented process), cross-training and pairing (deliberately spreading knowledge to a second person even when it's slower in the moment), rotation of responsibilities, and reducing single points of access. For an individual, being the sole holder of critical knowledge can feel like job security, but it's a double-edged position: it makes you hard to replace, but also hard to promote or give time off, since the team can't function without you sitting exactly where you are.