Because a company often uses machines over time that may vary in age, specification, processing speeds and set-up times, it is unlikely that they are (completely) interchangeable. As a result, it can be necessary to group semi-interchangeable machines. To evaluate the influence of this on shop performance, the degree of machine inter-changeability was studied at three levels, namely: 1, 0.2 and 0. These values are related with the percentage of jobs that can be performed in either of the two machines of the work centre 1 100% (interchangeable machines), 20% (semi-inter changeable) and 0% (no interchangeable).