Control charts based on the number of consecutive conforming items between two successive nonconforming items for the near zero-nonconformity processes

Monitoring the number of consecutive conforming items between two successive nonconforming items is an alternative approach of monitoring the fraction nonconforming. For the near zero-nonconformity processes, taking the number of consecutive conforming items between two successive nonconforming items as an object controlled and the average number of nonconformity (ANNC) as a comparison standard, the cumulative sum (CUSUM) chart and the exponentially weighted moving average (EWMA) chart are established. Comparisons with the two-stage control chart for the high-yield processes proposed by Chan, Xie and Goh (1996, International Journal of Reliability, Quality and Safe Engineering, 4, pp. 149-165) are then made, and it is shown that for the near zero-nonconformity processes the CUSUM chart and the EWMA chart, both based on the number of consecutive conforming items, are more efficient than the two-stage control chart; the ANNC of the former with a zero initial value is only one-half to one-third of the latter. Finally, a comparison of the CUSUM chart and the EWMA chart, both based on the number of consecutive conforming items, is made.