Use an Individuals (I) chart to assess process location. The I chart consists of the following:

·    Plotted points, which represent the subgroup averages.

·    Center line (green), which is the overall process average (average of all subgroup averages).

·    Control limits (red), which are located 3 s above and below the center line, and provide a visual means for assessing when the process is out of control. The limits, which represent the amount of variation expected for the subgroup averages, are calculated using the average of the moving range of the subgroup averages.

Minitab conducts up to eight tests for special causes for the I 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 I chart for the injection molding data can be summarized as follows:

·    The lower and upper control limits are 11.505 and 12.694, respectively. Therefore, the subgroup averages are expected to fall between 11.505 and 12.694. The center line (overall process average) is 12.099.

·    No subgroups failed the tests for special causes on the I chart; therefore there is no evidence for lack of control in process location.