An Improved p Chart Through Simple Adjustments

Summary: The p chart for monitoring the fraction, p, of nonconforming products requires very large sample sizes to achieve any power in detecting decreases in p, and the false alarm for probabilities differ considerably from the nomial normal values. Two alternative charts, namely, the chart based on discrete probability integral transformations and the chart based on the acsine transformation, can greatly reduce the severity of the above problems. We show in this paper that with simple adjustments to the control limits of the p chart, one can achieve equal or even better improvement, while still working on the original scale. In particular, using the simple adjustments is equivalent to using the exact binomial distribution to design a chart. Some guidelines for choosing an appropriate chart are also given.