Bootstrap lower confidence limits for the process capability indices Cp, Cpk and Cpm

The capability indices are widely used by quality professionals as an estimate of process capability. Many process indices have been proposed and developed with Cp, Cpk and Cpm among the most widely used. More recently, techniques have been developed to construct lower 95 percent confidence limits for each index. These techniques are based on the assumption that the underlying process is normally distributed. The non‐parametric but computer intensive method called Bootstrap is utilized and the Bootstrap confidence limits are calculated for these indices. A simulation using three distributions (normal, log‐normal and chi‐squared) was conducted and a comparison was made of the performances of the Bootstrap and the parametric estimates.

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