The SHEWHART Procedure

The Index $C_{pk}$

The process capability index $C_{ pk}$ is computed as

\[ C_{pk} = \min (USL- \overline{\overline{X} }, \overline{\overline{X}} - LSL)/ 3\hat{\sigma } \]

If you specify only USL, the index $C_{pk}$ is computed as

\[ C_{pk} = (USL- \overline{\overline{X}} ) / 3\hat{\sigma } \]

and if you specify only LSL, the index $C_{pk}$ is computed as

\[ C_{pk} = (\overline{\overline{X}} - LSL) / 3\hat{\sigma } \]