Group Runs Double Sampling np Control Chart for Attributes

This paper proposes a group runs (GR) double sampling (DS) np chart to detect increases in the fraction of non-conforming units. It combines the charting statistics of the DS np chart and an extended version of the CRL chart. The performance of the proposed GR DS np chart is evaluated and compared with other attribute charts, namely, the np, GR np, DS np, synthetic DS np, variable sample size (VSS) np, exponentially weighted moving average (EWMA) np, and cumulative sum (CUSUM) np charts, in terms of the average run length (ARL) criterion. The ARL result showed that the optimal GR DS np chart generally performs better than the optimal version of the charts under comparison, for detecting increases in the fraction of non-conforming units, for most shift sizes. The optimal charting parameters that simplify the implementation of the GR DS np chart are provided. The implementation of the proposed chart is illustrated with an example. Based on the significant improvement in the ARL performance, the GR DS np chart is a viable substitute of existing np-type charts for the detection of increases in the fraction of non-conforming units.