Identifying shifts (and other patterns)

Effective use of control charts requires periodic revision of control limits when changes to the process cause a reduction in variability or shift to a new operating point.

  1. On the Analyse-it ribbon tab, in the Process Control group, click Apply Rules, and then click Montgomery.
    In the analysis task pane, the Control Chart panel is displayed.
  2. In the Rules grid, clear the check boxes next to rules 4,6, 7 and 8.
  3. Click Recalculate.

The standard Shewhart control limits are good at detecting large shifts but are not sensitive to smaller shifts. Applying advanced out-of-control rules has identified a shift to a new process mean towards the end of the installation period suggesting that the process was moving to a new phase.

Xbar-R control chart with Nelson rules(click to enlarge)