This article considers the np x chart proposed by Wu et al. (2009) to control the process mean, as an alternative to the use of the <img src="/img/revistas/prod/2011nahead/AOP_T6_0002_0527_x.jpg" />chart. The distinctive feature of the np x 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 np x 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 np x chart. In general, the np x chart requires samples twice larger (the standard np chart requires samples five or six times larger) to outperform the <img src="/img/revistas/prod/2011nahead/AOP_T6_0002_0527_x.jpg" />chart.
Discriminating limits; np x chart; First order autoregressive model AR (1); Quality control; Attribute and variable control chart; Attribute inspection
