Is where you are at the end of the proverbial dayĮxamples of the Difference Between Cpk and Ppkįor illustration, let's consider a data set where 5 measurements were taken every day for 10 days.Theoretically includes both the variation within subgroups and also the shift and drift between them.Accounts for the OVERALL variation of all measurements taken.Is sometimes referred to as the potential capability because it represents the potential your process has at producing parts within spec, presuming there is no variation between subgroups (i.e.Does not account for the shift and drift between subgroups.Only accounts for the variation WITHIN the subgroups.Without boring you with the details surrounding the formulas for the standard deviations, think of the within standard deviation as the average of the subgroup standard deviations, while the overall standard deviation represents the variation of all the data. The only difference lies in the denominator for the Upper and Lower statistics: Cpk is calculated using the WITHIN standard deviation, while Ppk uses the OVERALL standard deviation. If we look at the formulas for Cpk and Ppk for normal (distribution) process capability, we can see they are nearly identical: The goal of capability analysis is to ensure that a process is capable of meeting customer specifications, and we use capability statistics such as Cpk and Ppk to make that assessment. For example, if you sample 5 items every hour, your subgroup size would be 5. Therefore, the measurements that make up a subgroup should be taken from a similar point in time. Subgroups are meant to represent a snapshot of your process. Rational SubgroupsĪ rational subgroup is a group of measurements produced under the same set of conditions. Poofy hairdos styled with mousse, shoulder pads, and leg warmers aside, let’s start by defining rational subgroups and then explore the difference between Cpk and Ppk.

It’s like the '80s pop duo Wham!, where Cpk is George Michael and Ppk is that other guy.

Back when I used to work in Minitab Tech Support, customers often asked me, “What’s the difference between Cpk and Ppk?” It’s a good question, especially since many practitioners default to using Cpk while overlooking Ppk altogether.

Cpk Calculations In Excel Free Software