Some procedures for decision making in controlling high yield processes

Statistical process control based on the cumulative counts of conforming items instead of occurrences of non‐conformances has proved to be useful for the manufacture of high‐quality products. In this paper we study some interesting and useful issues of using cumulative counts in practice. The general problem in the control of high‐quality products is first discussed, which leads to the calculation of the probability that a process is out of control when a single non‐conforming item is detected. A decision graph is introduced with which we can easily judge whether the process is out of control when a non‐conforming item is observed after a number of conforming ones, accompanied by the certainty level of this judgement. Next, using the equivalence of information on one non‐conforming item found in a number of inspected items to that on zero non‐conforming items in a smaller sample, we give a method for choosing a new starting point to reset the cumulative count after the detection of the non‐conforming item. The procedures are all statistically justified and have the practical merit of simplicity in actual applications.