Statistical Process Control (SPC) Resource Center

Five Steps: Setting Up a Variables Control Chart

The five steps for setting up X-bar & R control charts are:

1. Collect and Calculate Subgroup Data

  • Collect (at least) 20 subgroups of data from the process.

2. Calculate the Centerlines and Control Limits

  • The formulas for calculating the centerlines and control limits are given in Appendix 1. The control chart factors you’ll need for the limits can be found in Appendix 2.
  • Set the scales for the charts.
  • Add the centerlines and control limits.

3. Plot the Data

  • Plot on both the X-bar and the R Charts.

4. Interpret the Control Chart

  • If no points are outside the limits and there are no unusual patterns, the process is stable.
  • If more than two points are outside of the limits, it is not stable.
  • If one point is outside the limits, drop it, recalculate the centerline and limits, and replot the data. In this case, the process is stable if all points are within the control limits.

5. Take Action – Use the calculated limits if the process is stable.

  • Improve the process if it is not stable.
  • You cannot use a control chart on an unstable process.