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Abstract:
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Process capability analysis assesses how well a process produces product that is within specification. The standard process capability indices, Cp, Cpk, Cpl, and Cpu are metrics that relate process performance to specification limits that are popular and widely used in industry. Careful attention must be given to assure these indices are used correctly. A process must be in statistical control for these indices to have any meaning, and in order to make inferences about process conformance they require the assumption that the measurements come from a normal distribution. Unfortunately, non-normally distributed process measurements frequently occur in industry. This study will compare the performance of two methods for calculating process capability indices when process measurements come from a non-normal distribution, the percentile (ISO) method and the Z-score (Bothe) method. We will make assumptions about the distribution of the data and find the percentiles used in these indices based on the fitted distribution. We will study the performance of both methods for a variety of distributions and sample sizes.
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