ABSTRACT
Control charts are basic tools for quality assurance. They provide a graphical means to demonstrate statistical control, monitor a measurement process, diagnose measurement problems, document measurement uncertainty, and generally aid in methodology development. The concept of control charts was first developed in 1934 by Walter Shewhart to describe the outputs of manufacturing processes. Control limits can be based on established limits or experimentally established ones. For manufactured goods, the limits could be based on acceptability of product with the central line as a desired value and the control limits as permissible tolerances. The control samples must have a high degree of similarity to the actual samples analyzed. Otherwise, one cannot draw reliable conclusions about the performance of the measurement system on test samples from its measurement of control samples. The required frequency of measurement of control samples will depend on a number of factors, first of which is the known stability of the measurement process.
