Six Sigma Glossary from MiC Quality
Process Capability and Performance
 Capability Ratio

The reciprocal of Cp.

 Cp

Where:

 USL, LSL Upper and Lower Specification Limits sR The process standard deviation. It must be estimated by the range method. See Process Performance

A minimum Cp value of 1.67 is usually required.

 Cpk

A measure used to predict actual process performance:

Where:

 m Process mean

 Cpm

The Taguchi Capability Index combines variability and distance from the target into one measure (related to the 'Signal to Noise Ratio' concept):

Where:

 T Target value

 Long Term Capability

The Process Performance Indices Pp and Ppk are often referred to as 'long term' process capability. This is because the standard deviation used in their calculation is calculated using the 'Root Mean Square Error' (RMSE) method which is affected (inflated) by shifts in the process mean. The standard deviation used in the Process Capability calculation is calculated using the Range Method which is not affected by changes to the process mean.

The Process Performance is generally lower than the Process Capability. However there is much confusion and debate about the meaning of short and long term capability, so use these terms with care and be sure that everybody has the same understanding.

 Natural Process Limits

The 'Natural Process Limits' are at plus and minus three standard deviations from the target. If the natural tolerance limits are within the specification limits, the Cp value is greater than 1.

 Process Capability

Process Capability is a method used to demonstrate that a process is 'capable' of achieving the required tolerances (specifications). It is a routine requirement for all processes in the automotive industry, among others.

 Pp

See Process Performance Index

 Ppk

See Process Performance Index

 Process Performance Index

The Process Performance Indices Pp and Ppk are calculated in the same way as Cp and Cpk, except that the standard deviation is calculated using the RMSE method, not the Range Method

Pp and Ppk are measures of the long term performance of a process, whereas Cp and Cpk measure the short term performance.

 Short Term Capability

See Long Term Capability

 Specification Limits

The limits imposed by the designer, or the customer. The onus is on the supplier to ensure that the process can achieve the specifications to the required process capability value.

 Tolerance Limits

See Specification Limits

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