Process Control and Product Quality: The C⊂p and C⊂pk Revisited

Much emphasis in recent years has been placed on evaluating product quality by use of the C⊂p and C⊂pk indices. These capability indices, particularly the latter, have much appeal as they seemingly wrap up quality into the calculation of a single number. The purpose of this article is to explain the statistical essentials involved in using the C⊂p and C⊂pk indices to assess product quality. Having discussed confidence intervals, definitions and provided some quantitative results, caution is urged in using these indices in practice. Some practical suggestions provide a climax.