Optimal np Control Charts with Variable Sample Sizes or Variable Sampling Intervals

Many researches have shown that the adaptive control charts are more effective than the traditional static ones in detecting process shifts. This paper develops an algorithm for the optimization designs of the Variable Sample Size (VSS) np chart and the Variable Sampling Intervals (VSI) np chart for monitoring process fraction nonconforming p. The properties of the VSI and VSS np charts are measured by the steady-state Average Time to Signal (ATS). The performance of these adaptive np charts are studied and compared with the static np charts. It is found that the adaptive np charts do improve the effectiveness significantly, especially for detecting small or moderate process shifts.