Overview: What is capability? 

Capability is the statistical comparison of the Voice of the Customer (specs, requirements, or expectations of your customer or organization) and the Voice of the Process (control limits from your control chart). The goal of capability is to come up with a quantifiable measure of how well you are meeting your customer or organizational expectations.

There are several different metrics for measuring capability. One of the most common is Cpk or Capability Index. Other calculated metrics include; Cp, Pp and Ppk. Cp and Cpk are often referred to as short-term measures of capability. Pp and Ppk are referred to as long-term measures of capability. 

Process capability, Cpk, is important because it indicates whether a process has the potential to meet a specification. Process performance, Ppk, is important because it indicates how well the actual process has performed over time. Below are the formulas for Cpk and Ppk:

The difference between the two formulas is in the standard deviation used in each formula. Cpk uses a standard deviation estimated from the Rbar on an R control chart. Ppk uses the calculation for standard deviation of all the data.

An industry example of capability 

The graphic below shows the capability output from a statistical software package. Since the Cpk and Ppk are both below 1, you can say your process is not capable of meeting the desired specifications. Likewise, since the Cp and Pp values are about the same as Cpk and Ppk you can state that centering is not an issue. Finally, the Cpk and Ppk are about the same values which would indicate the process is in statistical control. The goal here would be to reduce the overall variation so the process can be capable of meeting the specifications.

Frequently Asked Questions (FAQ) about capability

What are the most common measures of process capability? 

The four most common measures of process capability are Cp, Cpk, Pp and Ppk. 

What is the difference between Cp and Cpk? 

Cp has the assumption your process is centered between your upper and lower specifications. If your process is not centered, use Cpk. If you only have a one-sided specification, you would use Cpk since Cp cannot be centered between specifications if you only have one. 

What is the most critical assumption about process capability measures? 

Process capability measures are only meaningful if the underlying process is in statistical control, that is, exhibiting common cause variation. You can use a control chart to determine if your process is in statistical control.