The Cp and Cpk are well-known capability indices commonly used to ensure that a process spread is as small as possible compared to the tolerance interval (Cp), or that it stays well within specifications (Cpk). Yet another type of capability index exists: the Cpm, which is much less known and used less frequently.

The main difference between the Cpm and the other capability indices is that the bias from the target is directly taken into consideration in the Cpm. The bias is the difference between the process average and the target.

In Minitab, as soon as a target is entered into a capability analysis, a Cpm gets calculated automatically. To obtain a good Cpm, a process must be right on target, whereas to obtain a satisfactory Cpk value, a process simply needs to stay well within specifications. This might seem to lead to the same conclusion; however, there’s an obvious and major difference: With the Cpm, a direct penalty is incurred for not being exactly on target.

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