Use and S charts to draw a control chart for subgroup means ( chart) and a control chart for subgroup standard deviations (S chart) on one page. Interpreting both charts together allows you to track both process center and process variation and detect the presence of special causes.

·    An in-control process exhibits only random variation within the control limits.

·    An out-of-control process exhibits unusual variation, which may be due to the presence of special causes.

The S chart must be in control in order to interpret the chart because the control limits are calculated considering both process spread and center.

·    If the S chart is out of control, then the control limits on the chart will be inaccurate and may falsely indicate an out-of-control condition. In this case, the lack of control will be due to unstable variation rather than actual changes in the process center.

·    When the S chart is in control, you can be sure that an out-of-control chart is due to changes in the process center.

A canning company wants to assess whether or not its can-filling process is in control. Fifteen subgroups of ten cans each were collected at fifteen-minute interval across two shifts for a single day's production.

To minimize the within-subgroup (can-to-can) variation the ten samples for a given subgroup were gathered in a short period of time. Minimizing this variation is important because within-subgroup variation is used to establish the control limits for the -S chart.

Data: Canning.MTW (available in the Sample Data folder).