A synthetic control chart for detecting fraction nonconforming increases

In this paper we propose a new control chart, the synthetic control chart, for the detection of increases in the fraction nonconforming (rho). The ope ration, design, and performance of this control chart are described. The sy nthetic chart is a combination of the np chart and the conforming run lengt h chart. It provides the user with some freedom in adjusting the control ch art parameters so that the out-of-control average time to signal can be min imized. Numerical tests have indicated that the synthetic chart has a highe r power for detecting process shifts in the fraction nonconforming than bot h the np chart and the conforming run length chart. The improvement is sign ificant for all cases, resulting in a 50% or greater reduction of the out-o f-control average time to signal. The application of the synthetic chart ma y therefore substantially enhance the effectiveness of statistical process control for attributes.