Glossary
Also: Truck factor
The number of people who would have to disappear before a system can no longer be developed or operated. For many legacy systems it is one.
The term is deliberately blunt: how many people would have to be hit by a bus before the project stops. What is meant is knowledge that exists in one head and nowhere in writing.
A bus factor of one is not a staffing problem, it is an operational risk with a price tag. It becomes visible when that person resigns, falls ill or takes a holiday.
How you notice it
Frequently asked
By having a second person ship a real change, accompanied but not demonstrated. The documentation then emerges as a by-product from the places where they stumbled. That is more effective than any manual, because it tests instead of describing.