Z benchmark indices are an alternative to Cp and Pp indices. They are the definition of the sigma capability of a Six Sigma process.
All of the indices assume a normally distributed process quality characteristic with the parameters specified by the process mean and sigma. The process sigma is either the short-term or long-term sigma estimate.
Various indices measure how the process is performing against the specification limits:
| Index | Description |
|---|---|
| < LSL | The number of sigma units from the process mean to the lower specification limit. |
| > USL | The number of sigma units from the process mean to the upper specification limit. |
| < SL > | The number of sigma units from the process to mean to the point if all nonconforming units are put in one tail of the distribution. |
Z shift is the difference between the short-term and long-term indices. The larger the Z shift, the more scope there is to improve the process by eliminating shifts and drifts in the process mean. Some industries define the sigma capability of a process as the long-term Z benchmark + a 1.5 Z shift. Meaning a process with a long-term Z benchmark of 4.5 is quoted as a Six Sigma process. It is best to avoid such rules and directly measure the short-term and long-term Z benchmark capability.
From the Statistical Reference Guide for Analyse-it 6.24.0: https://analyse-it.com/docs/user-guide/process-capability/z-benchmark
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