Example of Xbar-R Chart

A quality engineer at an automotive parts plant monitors the lengths of camshafts. Three machines manufacture camshafts for three shifts each day. The engineer measures five camshafts from each machine during each shift.

The quality engineer creates an Xbar-R chart for each machine to monitor the camshaft lengths.

  1. Open the sample data, CamshaftLength.MTW.
  2. Open the Xbar-R Chart dialog box.
    • PC: STATISTICS > Control Charts > Continuous > Xbar-R
    • Mac: Statistics > Continuous > Xbar-R
  3. From the first drop-down list, select Data are in one column for all subgroups. In Data column, enter Machine 1.
  4. From the second drop-down list, select Subgroup IDs are in a column. In Subgroup ID column, enter Subgroup ID.
  5. Click OK.

Interpret the results

The engineer looks at the R chart first because, if the R chart shows that the process variation is not in control, then the control limits on the Xbar chart are inaccurate.

The R chart shows that the process variation is in control. No points are out of control, and all the points fall within the control limits in a random pattern.

The Xbar chart show that Machine 1 may not be in control. One point is out-of-control point.

Method
Parameters
Mean
StDev
Both parameters estimated
Tests for Special Causes
Results for Xbar Chart
Test
Description
Failed Subgroups
1 point > 3 standard deviations from the center line
8
Results for R Chart
Test
Description
Failed Subgroups
1 point > 3 standard deviations from the center line
None
By using this site you agree to the use of cookies for analytics and personalized content.  Read our policy