Use moving range (MR) charts to assess whether process variation is in control when your data are individual measurements. The MR chart consists of:

·    Plotted points for the moving range, which is the absolute value of the difference between two consecutive points.

·    Center line (green), which is the average of all the moving ranges.

·    Control limits (red), which are located above and below the center line and provide a visual display for the amount of variation expected in the moving ranges.

Minitab conducts up to four tests for special causes for the MR chart, which detect points beyond the control limits and specific patterns in the data. Points that fail are marked with a red symbol and the number of the failed test. Complete results are printed in the Session window. A failed point indicates that there is a nonrandom pattern in the data that may be the result of special-cause variation. These points should be investigated.

The MR chart for the liquid detergent data can be summarized as follows:

·    The lower and upper control limits are 0 and 0.4983, respectively. Therefore, the moving ranges are expected to fall between 0 and 0.4983. The center line is 0.1525.

·    None of the individual observations fall outside the control limits. Furthermore, the points inside the limits display a random pattern. This MR chart does not provide any evidence for lack of control. Thus, the process variation is in control.