An np control chart for monitoring the mean of a variable based on an attribute inspection

This article proposes a new np control chart, called the npx chart, that employs an attribute inspection (inspecting whether a unit is conforming or nonconforming) to monitor the mean value of a variable x. The distinctive feature of the npx chart is using the statistical warning limits to replace the specification limits for the classification of conforming or nonconforming units. By optimizing the warning limits, the npx chart usually outperforms the X¯ chart on the basis of same inspection cost. In addition, the npx chart often works as a leading indicator of trouble and allows operators to take corrective action before any defective is actually produced.