QLI – a valuable index on assurance of quality for customers of

QLI – a valuable index on assurance of quality
for customers of process industries
D.D. Hanagal1 and J. Samajdwar2
1
Professor, Department of Statistics, University of Pune, Pune-411007, India
2
Consultant, Application of SQC to Process Industries, Pune, India
Abstract:
QLI (Quality Loss Index) is a process performance index following Taguchi’s Quality Loss
Function approach (ref. [5] and [6]). Ppk is the existing process performance Index and is equal to Cpk
when the process is under statistical control. Herman, J.T. in 1989 (ref. [3]) had expressed that the
‘process performance index (PPI)’ has more value than ‘process capability index (PCI)’ for customers of
process industries. Process performance indices Ppk and QLI are compared here for small sample sizes.
Ppk and QLI are not identical from their approach. Process performance is better when Ppk is higher but
QLI is lower. Both Ppk and QLI values on standard normal population are converted to a common
platform, the corresponding process sigma value. Performances of Ppk and QLI are compared here on
these converted values.
It is observed that from application and operational point of views QLI is superior to Ppk
irrespective of sample size. For distributional properties, QLI has a well defined gamma distribution but
Ppk values do not follow a standard distribution. Possibility of calculation of CQLI (composite QLI) and
control a process with many process variables using CQLI is the most attractive point for use of QLI.
Average value of QLI is closer to the target compared to that of Ppk. Ppk has a higher spread of values
compared to that of QLI. Thus QLI is a superior performance Index to Ppk because of its various
applications, operational ease, distributional pattern and spread around the target value. One may
therefore, conclude that QLI is more valuable to customer than Ppk. Process capability indices Cp, Cpk
and Cpm etc. continue to perform their respective responsibilities but customer gets more information
from QLI.
Paper Category: Quality by Statistical Methods / Statistical Process Control
Keywords:
Process Performance Index, Ppk, QLI, CQLI, QLIns, QLIhigh, QLIlow.
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