In software development, a software project's bus factor (also known as truck factor, or bus/truck number or lottery factor) is a measurement of the concentration of information in individual team members. The bus factor is the total number of key developers who would need to be incapacitated (for example, by getting hit by a bus/truck) to send the project into such disarray that it would not be able to proceed. In this event, the project would retain information (such as source code or other systems) with which no remaining team member is familiar. A high bus factor, where many individuals know enough to carry on in the event of the loss of some of them, means that many developers would need to be removed before the project would necessarily fail.
"Getting hit by a bus" could take many different forms, including anything that suddenly and substantially prevented the individual from working on the project. This could be a person taking a new job, having a baby, changing their lifestyle or life status, or literally getting hit by a bus: the effect would be the same. The term was commonplace in business management by 1998 and was used in mental health in the same year. It was seen in software engineering papers in Association for Computing Machinery and Information Systems Frontiers by 1999, in engineering by 2003, and the Debian project in 2005.
- Coplien, John -Pair Programming Illuminated. Quote: "How many or few would have to be hit by a truck (or quit) before the project is incapacitated?"
- Simon, Robert (17 May 1998). The Mental Health Practitioner and the Law: A Comprehensive Handbook. Harvard University Press. p. 69. ISBN 0-674-69721-9.
- Redmond, Matthew C.; Newton, Paul (2003). "Integrating GIS in the Engineering, Planning and Design Processes".
- Reinholdtsen, Petter (2005-11-11). "Re: Resignation and uploads". https://lists.debian.org/debian-devel/2005/11/msg00801.html.
- McLay, Michael (1994-06-29). "If Guido was hit by a bus?". http://legacy.python.org/search/hypermail/python-1994q2/1040.html.
- Michele Marchesi, Giancarlo Succi, Don Wells, James Donovan Wells, Laurie Williams (2003). Extreme Programming Perspectives. Boston u. a.: Addison-Wesley. ISBN 0-201-77005-9.
- Laurie Williams, Robert Kessler (2002). Pair Programming Illuminated. Boston u. a.: Addison-Wesley. ISBN 0-201-74576-3.
- Kent Beck (2000). Extreme Programming. Das Manifest (in German). s. l.: Addison-Wesley. ISBN 3-8273-2139-5.
- Poisonous People, a talk that includes (among other topics) discussion of bus factor and how to increase it