APPROACHES TO THE PROCESS CAPABILITY ANALYSIS IN THE CASE OF NON- NORMALLY DISTRIBUTED PRODUCT QUALITY CHARACTERISTIC

Process capability analyses, which characterize the ability of processes to produce products meeting the quality criteria, are now a standard part of products and processes quality planning and improvement. Automotive industry considerably contributes to their wide application, because its suppliers must demonstrate the capability of their processes. Standard process capability analysis procedure applies a number of assumptions. One of them is the assumption of normal distribution of given quality characteristic that may not be met in real conditions. The article deals with possible causes of not meeting this assumption and it highlights the fact that in many cases it may not be related to the nature of the quality characteristic distribution. For situations where non-normality is the natural behaviour of the product quality characteristic, various approaches that make it possible to evaluate the process capability are analysed. In a concrete example, the results of the application of selected approaches are compared and discussed.