ome processes are such that forming subgroups is impossible or undesirable. For example, ongoing
accounting data such as weekly, monthly, or quarterly sales totals are impossible to subgroup. In these
cases, individual measurements are typically plotted in an individuals chart, sometimes called an X chart.
Sometimes, the moving range (i.e., the difference between the current period and the previous period) is plotted in a
moving range chart, known as an MR chart, and presented in combination with the individuals chart (which situation
we shall call a combination chart or an XMR). In general, the X and MR portions are used to monitor the process
mean and variability, respectively. On other occasions, the individuals chart is used alone. There is much
disagreement as to whether the individuals chart alone or an XMR is best. Moreover, if the XMR is employed, there
is the practical matter of the design of the chart, i.e., what combination of control limits is optimal.