Bus Factor

Bus Factor (also known as Truck Factor or Lottery Factor) is an empirical metric that answers the question: “How many team members have to be simultaneously lost before the project grinds to a halt?”. The somewhat macabre name comes from the hypothetical scenario of a colleague hit by a bus — but it applies equally to extended leave, illness, resignations, or transfers.

How to calculate it #

There’s no exact mathematical formula, just a reasoned estimate starting from a few questions:

• Who is the only person who knows how to configure the production cluster?
• Who is the only person who knows the functional domain of a given area?
• Who wrote the most critical piece of code without documenting it?
• Who maintains the relationship with a key stakeholder on the customer side?

If the answer to each question is “one person”, the bus factor is 1 on that competence. The team’s bus factor is the minimum across all individual critical-competence bus factors.

Typical values #

• Bus factor = 1: critical risk. A single person holds knowledge that would block the project. Common in small teams or “guru-style” work.
• Bus factor = 2: fragile. Covered if one person is out, but if both are gone the project stalls.
• Bus factor ≥ 3: resilient. Knowledge is distributed enough to absorb multiple absences.

The pragmatic target in real projects is to keep the bus factor ≥ 3 on truly critical competences, accepting lower values on more peripheral areas.

How to raise the bus factor #

Four tools, all low-cost but requiring calendar time:

• Minimal documentation: not encyclopedias, but 2-5 page operational runbooks on critical procedures
• Pair working: two people on the same activity, alternating between “hands on keyboard” and “observes and asks”
• Rotation: whoever has always done X moves to Y this month, and vice versa. Even just for a week
• Recurring knowledge transfer: 30 minutes on the calendar every week on a specific topic, recorded

Signs that the bus factor is low #

• When one person goes on leave, the team visibly slows down
• Some activities are systematically assigned to the same person
• A critical procedure has never been documented
• The lead is the only one who knows the “why” of certain architectural choices