Monitoring linearity of measurement gauges

In the quality control literature, most of the control charts are developed for monitoring the product quality characteristics or process parameters. However, it is also important to monitor measurement gauges that measure the product quality characteristics because a malfunctioned gauge will result in false out-of-control or insensitive signals. In this article, we develop a Shewhart chart for monitoring the linearity between two measurement gauges. An in-control linearity will ensure consistency of measurements and provides evidence that the gauges are functioning properly. Consistency of measurements is an important indicator of quality in quality control and process improvement activities. We provide a simple design procedure to determine the control limits. When the chart parameters are estimated, the minimum number of samples required is also given. An application is demonstrated with a real manufacturing data set.