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Six Sigma metrics based on lognormal distribution for life tests

Ravichandran Joghee (School of Engineering, Amrita Vishwa Vidyapeetham, Coimbatore, India)

International Journal of Quality & Reliability Management

ISSN: 0265-671X

Article publication date: 10 May 2019

Issue publication date: 23 September 2019

255

Abstract

Purpose

The purpose of this paper is to propose an approach for studying the Six Sigma metrics when the underlying distribution is lognormal.

Design/methodology/approach

The Six Sigma metrics are commonly available for normal processes that are run in the long run. However, there are situations in reliability studies where non-normal distributions are more appropriate for life tests. In this paper, Six Sigma metrics are obtained for lognormal distribution.

Findings

In this paper, unlike the normal process, for lognormal distribution, there are unequal tail probabilities. Hence, the sigma levels are not the same for left-tail and right-tail defects per million opportunities (DPMO). Also, in life tests, while left-tail probability is related to DPMO, the right tail is considered as extremely good PMO. This aspect is introduced and based on which the sigma levels are determined for different parameter settings and left- and right-tail probability combinations. Examples are given to illustrate the proposed approach.

Originality/value

Though Six Sigma metrics have been developed based on a normality assumption, there have been no studies for determining the Six Sigma metrics for non-normal processes, particularly for life test distributions in reliability studies. The Six Sigma metrics developed here for lognormal distribution is new to the practitioners, and this will motivate the researchers to do more work in this field of research.

Keywords

Citation

Joghee, R. (2019), "Six Sigma metrics based on lognormal distribution for life tests", International Journal of Quality & Reliability Management, Vol. 36 No. 9, pp. 1477-1489. https://doi.org/10.1108/IJQRM-05-2018-0135

Publisher

:

Emerald Publishing Limited

Copyright © 2019, Emerald Publishing Limited

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