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Abstract

HO, Linda Lee  and  COSTA, Antonio Fernando Branco. Monitoring a wandering mean with an np chart. Prod. [online]. 2011, vol.21, n.2, pp.254-258.  Epub June 03, 2011. ISSN 0103-6513.  http://dx.doi.org/10.1590/S0103-65132011005000027.

This article considers the npx chart proposed by Wu et al. (2009) to control the process mean, as an alternative to the use of the chart. The distinctive feature of the npx chart is that sample units are classified as first-class or second-class units according to discriminating limits. The standard np chart is a particular case of the npx chart, where the discriminating limits coincide with the specification limits and the first (second) class unit is the conforming (nonconforming) one. Following the work of Reynolds Junior, Arnold and Baik (1996), we assume that the process mean wanders even in the absence of any specific assignable cause. A Markov chain approach is adopted to investigate the effect of the wandering behavior of the process mean on the performance of the npx chart. In general, the npx chart requires samples twice larger (the standard np chart requires samples five or six times larger) to outperform the chart.

Keywords : Discriminating limits; npx chart; First order autoregressive model AR (1); Quality control; Attribute and variable control chart; Attribute inspection.

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