2015 QPRC

Process Capability Indices: Some Subtle Misconceptions

M. Z. Anis
SQC & OR Unit
Indian Statistical Institute
203, B. T. Road
Calcutta – 700 108
INDIA
E-mail: mohdzanis@yahoo.com

Abstract
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. The basic argument of the paper is that capability indices are powerful tools, but can inflict heavy damage if used incorrectly.

Key Words and Phrases: Loss function; Non-conformance; Performance of a process; Specification limits; Target; Variability.