Control Limits

The x-bar graph at the top of the chart has been suitably scaled, and the following lines drawn on it:

Similarly, the scale, together with the lines for UCLR, LCLR and the mean of the R values (R-bar) are shown on the R graph.

The position of each of these lines has been calculated from previously recorded data, in order to:

The area between the Upper and Lower Control Limits defines the anticipated "natural" spread of the process as the monitoring continues. This assumes, of course, that the process under investigation is in a state of:

The chart footings, below the recording boxes, are used for calculating control limits. Because those calculations are beyond the scope of this course, you can simply ignore that section.