Can a variable be monitored by an np chart effectively?

This article proposes a new np control chart, called the np_x 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 np_x 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 np_x chart usually outperforms the X chart considerably on the basis of the same inspection cost. In addition, the np_x chart often works as a leading indicator of trouble and allows operators to take corrective action before any defective is actually produced.