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Process Capability: Minding Your Cpk's

Tue, 12/02/1997 - 14:27
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The process potential index, or Cp, measures a process's potential capability, which is defined as the allowable spread over the actual spread. The allowable spread is the difference between the upper specification limit and the lower specification limit. The actual spread is determined from the process data collected and is calculated by multiplying six times the standard deviation, s. The standard deviation quantifies a process's variability. As the standard deviation increases in a process, the Cp decreases in value. As the standard deviation decreases (i.e., as the process becomes less variable), the Cp increases in value.

By convention, when a process has a Cp value less than 1.0, it is considered potentially incapable of meeting specification requirements. Conversely, when a process Cp is greater than or equal to 1.0, the process has the potential of being capable.

Ideally, the Cp should be as high as possible. The higher the Cp, the lower the variability with respect to the specification limits. In a process qualified as a Six Sigma process (i.e., one that allows plus or minus six standard deviations within the specifications limits), the Cp is greater than or equal to 2.0.

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Submitted by Pratheesh Soman on Mon, 11/25/2013 - 21:03

What if the specifications of

What if the specifications of the product is less than the Minimum Specification limit or if it is greater than the Maximum standard limit?

Then Cpk could be negative right?

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Submitted by Anonymous (not verified) on Sat, 11/07/2020 - 11:12

In reply to What if the specifications of by Pratheesh Soman

What if the specifications of

Cpk is considered zero in that case. Negative values by formula are just replaced by zero indicating that the process is not capable at all. This is true to best of my knowledge.

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