Process Capability Index

Definition of Process Capability Index:

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Cpk answers the question of how well your process is meeting your customer’s specifications. This question should be answered before you deliver your product or service to your customers rather than wait for them to let you know how well you are doing.

Overview: What is the Process Capability Index (Cpk)?

Cpk is the quantifiable comparison of the Voice of the Customer (specs, requirements, or expectations of your customer) and the Voice of the Process (control limits). The goal of Cpk is to calculate a quantifiable measure which lets you know whether you are meeting your customer expectations or not. The formula for Cpk is:

Cpk utilizes the customer specs and process variation, as measured by the standard deviation, but it also includes the location of the mean relative to the specs. Since the process mean may be off center, you will report out the minimum condition depending on whether the process has shifted closer to the upper spec or the lower spec.

The value of the Cpk  ratio can take on any value ranging from 0 to any positive or negative number. A value greater than positive 1 means the spec limit is far enough away that the process should be capable and be within specs. A value of 0 indicates the mean of the distribution sits on the spec so half the process will be out of spec. A negative value tells you the process mean is beyond the spec, resulting in more than 50% of the process being out of spec.

An industry example of Process Capability Index (Cpk)

The Black Belt (BB), assigned to help manufacturing deploy Six Sigma in the company, collected some data to show whether a product was capable of meeting a customer’s specs. The company was receiving multiple quality complaints so upper management was concerned. Here is the output from the statistical program the BB used. Notice the Cpk is less than 1 so the process is not currently capable of meeting the customer’s specs.

Frequently Asked Questions (FAQ) about Process Capability Index (Cpk)

1. Are there other measures of process capability besides Cpk?

Yes. Cp is a measure which assumes your process mean is centered between an upper and lower spec. You also have Pp and Ppk which are measures of long term process capability while Cp and Cpk are measures of short term capability.

2. What is the difference between Cp and Cpk?

Cp is a measure of process capability which assumes the process is centered within the specs. Cpk adjusts for a shift in the mean of the process and should be used when the process mean is not centered within the specs..

3. Can Cpk be used to compare different functions in my business?

Yes. Cpk is a generic measure of how well a function is doing relative to its required specs. By using Cpk to compare functions, you can state that manufacturing is doing better at meeting specs than sales if manufacturing’s Cpk is higher.

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