The bus factor is a measurement of the risk resulting from information and capabilities not being shared among team members, derived from the phrase “in case they get hit by a bus”. It is also known as the bread truck scenario, bus problem, beer truck scenario, lottery factor, truck factor, bus/truck number, or lorry factor.
The concept is similar to the much older idea of key person risk, but considers the consequences of losing key technical experts, versus financial or managerial executives (who are theoretically replaceable at an insurable cost). Personnel must be both key and irreplaceable to contribute to the bus factor; losing a replaceable or non-key person would not result in a bus-factor effect.
The term was first applied to software development, where a team member might create critical components by crafting code that performs well, but which also is unavailable to other team members, such as work that was undocumented, never shared, encrypted, obfuscated or not published. Thus a key component would be effectively lost as a direct consequence of the absence of that team member, making the member key. If this component was key to the project’s advancement, the project would stall.