On some subtle misconceptions about process capability indices

This paper describes some of the misconceptions about existing process capability indices (PCIs). Attention has been drawn to their drawbacks. It is shown that the indices have failed to guide the users in the right direction. This is perhaps due to the lack of consistency between the observed index value and the non-conformance levels. We give different examples to elucidate this point. It is also shown that the indices have failed to distinguish between on-target and off-target processes. Such a critical study of process capability analysis has to the best of our knowledge has not been attempted before. The basic argument of the paper is that capability indices are powerful tools, but can inflict heavy damage if used incorrectly. The paper will be welcomed by practitioners and users in the industry and management.