The frequency with which different part numbers are produced in a manufacturing process or system. 

If a machine is converted in such a sequence that each part number made with it is produced again after three days, the EPEx is three days. In general, it is good if the EPEx is as small as possible; then small quantities of each part are produced and inventories within the system are kept to a minimum. However, the EPEx of a machine depends on its changeover times and on the number of part numbers made with that machine. A machine with long changeover times (and large minimum batch sizes) that produces many different part numbers will inevitably have a large EPEx unless the changeover times or the number of different parts can be reduced.