A Synthetic Control Chart for Detecting Fraction Nonconforming Increases

Abstract

In this paper we propose a new control chart, the synthetic control chart, for the detection of increases in the fraction nonconforming (p). The operation, design, and performance of this control chart are described. The synthetic chart is a combination of the np chart and the conforming run length chart. It provides the user with some freedom in adjusting the control chart parameters so that the out-of-control average time to signal can be minimized. Numerical tests have indicated that the synthetic chart has a higher power for detecting process shifts in the fraction nonconforming than both the np chart and the conforming run length chart. The improvement is significant for all cases, resulting in a 50% or greater reduction of the out-of-control average time to signal. The application of the synthetic chart may therefore substantially enhance the effectiveness of statistical process control for attributes.