As an example, consider a group of six machines maintained by one repair person. When one machine breaks down, the source population is reduced to five, and the chances of one of the five breaking down and needing repair is certainly different than the chances of one out of six, as in the original size. If two machines are down with only four operating, the probability of another breakdown is again changed. Conversely, when a machine is repaired and returned to service, the machine population increases, thus raising the probability of the next breakdown.