The effects of interchangeability can often only be gleaned after the fact from the cause-and-effect insights
incorporated in the cost model. For example, two workers do the same type of work, but one is less skilled
and requires more inspection and rework. On the surface, these workers are interchangeable but, in reality,
interchanging will change the cost structure of the resource pool. The cost impact of interchangeability is
normally apparent in a historical analysis, but it is difficult to model since it is often unknown whether the
substitution will be feasible or acceptable.