Monitoring capability indices using run rules

Abstract

Purpose The purpose of this paper is propose a methodology for monitoring industrial processes that cannot be stabilized, but are nevertheless capable. Design/methodology/approach The proposed procedure uses the CP(u,v) family of capability indices proposed by Vännman (including the indices CPK, CPM, CPMK) combined with one‐sided two‐out‐of‐three and three‐out‐of‐four run rules strategies. Findings This paper introduces a new strategy, where capability indices are monitored in place of the classical sample statistics like the mean, median, standard deviation or range. Practical implications When doing a capability analysis it is recommended to first check that the process is stable, e.g. by using control charts. However, there are occasions when a process cannot be stabilized, but is nevertheless capable. Then the classical control charts fail to efficiently monitor the process position and variability. The approach suggested in this paper overcomes this problem. Originality/value The experimental results presented in this paper demonstrate how the new proposed approach efficiently monitors capable processes by detecting decreases or increases of capability level.

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