Comparison of pp control charts for low defective rate

It is well known that the conventional pp control chart constructed by the normal approximation for the binomial distribution suffers a serious inaccuracy in the monitor process when the true rate of nonconforming items is small. A similar problem also arises in the binomial confidence interval estimation. Adjusted confidence intervals are established in the literature to improve the coverage probability when the binomial proportion is small. In this paper, a new pp control chart based on an adjusted confidence interval is established, which can substantially improve the existing control charts when the nonconforming rate is small.