Synthetic double sampling np control chart for attributes

•A synthetic double sampling (DS) np chart is proposed.•The synthetic DS np chart integrates the DS np and the conforming run length (CRL) charts.•An optimization algorithm of the proposed chart for minimizing ARL1 is developed.•The proposed chart outperforms the existing np type charts for most of the cases.•The proposed chart can be implemented in production lines in manufacturing industries.

The quality of a product, based on the number of non-conforming items can be controlled using the np chart. This paper proposes a synthetic double sampling (DS) np chart which comprises two sub-charts, i.e. the DS np and conforming run length (CRL) sub-charts. For the zero-state case, the synthetic DS np chart surpasses its standard counterpart, i.e. the synthetic np and the basic DS np chart, and other np type charts like the standard np, combined synthetic and np (Syn-np), variable sample size (VSS) np, exponentially weighted moving average (EWMA) np and cumulative sum (CUSUM) np charts, for detecting increases in the fraction of non-conforming items p, for most shift sizes. The synthetic DS np chart also performs reasonably well in the steady-state case in comparison with other charts mentioned above. Thus, among the competing charts, the synthetic DS np chart stands out as one of the best charts.