S charts assess whether the process variation is in control. An S chart consists of the following:

·    Plotted points, which represent subgroup standard deviations, and provide an indication of the variation present within the subgroup.

·    Center line (green), which is the estimate of the process standard deviation (average of all the subgroup standard deviations).

·    Control limits (red), which are located above and below the center line, and provide a visual means for assessing when the process is out of control. The control limits, which represent the amount of variation expected for the subgroup standard deviations, are calculated using the variation within subgroups.

Minitab conducts up to four tests for special causes for the S 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 available in the Session window. A failed point indicates that there is a non-random pattern in the data that may be the result of special-cause variation. These points should be investigated.

The S chart for the canning data can be summarized as follows:

·    The lower and upper control limits are 0.00671 and 0.04057, respectively. Therefore, the subgroup ranges are expected to fall between 0.00671 and 0.04057. The center line (estimate of process standard deviation) is 0.02364.

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