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Process Capability Index

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Process Capability is required by TS 16949 and is part of the typical APQP process. Process Capability compares the output of an in-control process to the specification limits by using capability indices. The comparison is made by forming the ratio of the spread between the process specifications (the specification "width") to the spread of the process values, as measured by 6 process standard deviation units (the process "width").

Process Capability Indices - Examples: Cp, Cpk, and Cpm

A process capability index uses both the process variability and the process specifications to determine whether the process is "capable" We are often required to compare the output of a stable process with the process specifications and make a statement about how well the process meets specification. To do this we compare the natural variability of a stable process with the process specification limits. A capable process is one where almost all the measurements fall inside the specification limits. There are several statistics that can be used to measure the capability of a process: Cp, Cpk, Cpm. Most capability indices estimates are valid only if the sample size used is 'large enough'. Large enough is generally thought to be about 50 independent data values.

Typically:

Cp = Process Capability. A simple and straightforward indicator of process capability.

Cpk = Process Capability Index. Adjustment of Cp for the effect of non-centered distribution.

Pp = Process Performance. A simple and straightforward indicator of process performance.

Ppk = Process Performance Index. Adjustment of Pp for the effect of non-centered distribution.


See this animated visual of Process Capability - Cp vs. Cpk for a more detailed paper writing explaination and a visual animation which shows the relationship.

Elsmar Cove forum Process Capability discussion threads