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This is a unique case, where the Target isn’t the center of tolerance.

This Cpm is a good capability measure for one sided tolerances. In this scenario, Cpk is high but Cpm isn’t. In also such scenarios Cpm will be a relevant centering metric. For ex: customers are told that payment will be updated in 4 hours but the internal target is to get it done in 2 hours. However, in all service scenarios, there is a target value. In some cases, Cpk looses its significance. In most business processes, Service Levels have only an upper or lower specification. In this case, there is only a specification on one side. Cpk and Cpm Scenario 4 Cpk and Cpm Scenario 5 While this example is an extreme scenario with very liberal tolerance, it’s meant to illustrate the fact that Cpm is very sensitive to even small shift in mean from center, while Cpk isn’t. In this scenario Cpk value hasn’t really dropped much, but Cpm has crashed down. Out here, the process has liberal tolerance but a meagre shift from the target. Cpk and Cpm Scenario 3 ScCpk and Cpm Scenario 4 Process is centered but tolerance is liberal. Cpk and Cpm Scenario 2 Cpk and Cpm Scenario 3 Process is not centered and Cpk & Cpm both have reduced. Cpk and Cpm Scenario 1 Cpk and Cpm Scenario 2 Cpk and Cpm are high and nearly the same. Process is centered, mean and target are close to each other. Let’s now take a few scenarios to compare Cpk and Cpm values. To draw an analogy to plane landing, we are comparing the plane nose to the median line of the strip. This means, Cpm is a true process centering capability index, because it compares the difference between process mean and target value. Thus in addition to obtaining the USL and LSL from the customers, we also obtain the Target value. If you look at the formula of Cpm, it contains the target value. Now it’s look at Cpm and then draw comparisons. That is good but it’s only an indirect way of assessing the process center. In a way, this is an indirect method of assessing if the process centered to the tolerance.Īs an analogy, in order to know if the plane is landing exactly on the middle of the landing strip, we look at the distance of the plane’s nose from either edge of the strip. However it takes the lowest value of (USL – Process Mean) and (Process Mean – LSL). Target Value is the center value obtained from customer specification. If you closely observe the Cpk formula, it doesn’t take the actual Target value. Cpk Cpk Illustration and FormulaĬpk is considered as a measure of the process centering. In order for us to understand Cpm, let’s take a step back and understand Cpk. It is also called as Taguchi Capability Measure. In fact it is more robust than Cpk.Ĭpm is an advanced measure and it corrects some deficiencies in Cpk. Cpm is yet another interesting process capability metric that is not very popular.