During the various stages of the process of developing a software application there are many parameters that have to be taken care of. It is important to think about designs, layouts, specifications etc. beforehand. It is also important to delegate tasks to those developers who are most capable of finishing it with the highest productivity. Efficiency and resilience are important factors within a project.

Amongst other things the truck factor (or also bus factor) is a key figure for estimating risks within a project. It describes the possibility  for failure in a project if a certain number of developers would  drop out. It is described in Pair Programming Illuminated, Page 41 by John Coplien as followed:

“How many or few would have to be hit by a truck (or quit) before the project is incapacitated?”

The truck factor was proposed by one of the inventors of Extreme Programming, Kent Beck. The worst value is 1, which means each and every member of the development team is a specialist which is absolutely necessary. The perfect value 0 would describe that every project member is dispensable. The reality is usually somewhere in between.

To solve the issue, XP is proposing a pragmatic solution: All members of the project team are equally responsible for all parts of the project. This can be realized with pair programming in which the partners rotate heavily within all the pairs so the knowledge about the project will be quite quickly and evenly distributed amongst all the project members.