Know the truck factor of anything you cannot afford to lose
Severe knowledge concentration is the normal state rather than a symptom of neglect — measured across 133 popular, actively maintained projects, roughly two thirds could be incapacitated by two people leaving — so it has to be measured deliberately rather than noticed.
Do this firstGive every component an owner, and watch the drive-by edits
Count, per component, the smallest number of people whose departure would leave nobody who understands it. Then look at the components where that number is one.
The finding that makes this worth a page is not that knowledge concentrates — everyone suspects that — but how ordinary it is at the top of the distribution. These were popular, healthy, actively maintained projects, and most of them were two departures from trouble.
Be careful about what the evidence covers. It measures the condition, not its consequences, and the projects studied were open source. Treat a low number as a question worth asking rather than a defect in itself.
The decoy
A wiki page written by the one person who knows. It converts tacit knowledge into a document nobody has tried to use, which is a different and much weaker thing than a second person who has done the task.
Evidence
- Organizational Volatility and its Effects on Software Defects — II"recent departures from an organization were associated with increased probability of customer-reported defects" after controlling for size, coupling, release count and experience. Notably, the number of newcomers joining was NOT a significant predictor — the cost is in people leaving, not in onboarding.
- A Novel Approach for Estimating Truck Factors — II"87 systems (65%) have TF ≤ 2" — most popular, actively maintained open-source projects could be incapacitated by one or two departures. In 84% of valid survey answers respondents agreed the identified authors were the main authors of their system.
Last reviewed 2026-08-19.