Use and R chart to draw a control chart for subgroup means ( chart) and a control chart for subgroup ranges (R 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 R chart must be in control in order to interpret the chart because the control limits are calculated considering both process variation and center.

·     When the R chart is not in control, 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 R chart is in control, you can be sure that an out-of-control chart is due to changes in the process center.

An automobile assembly plant collected camshaft length measurements to assess the quality of the process. Five camshafts were measured from each of four shifts daily for five days. The five samples that comprise each subgroup were selected within a short period of time to minimize variation from one camshaft to the next.

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