Xbar-R Chart

Test Results - R Chart

R charts assess whether the process variation is in control. The tests for special causes detect points beyond the control limits and specific patterns in the data plotted on the chart.

· When a point fails a test, Minitab reports it in the Session window and marks it on the $image\xbar.gif$ chart. A failed point indicates that there is a nonrandom pattern in the data that may be the result of special-cause variation. These points should be investigated.

· When no points are displayed under the test results, no points failed the tests for special causes.

The R chart must be in control before you can interpret the $image\xbar.gif$ chart. If the R chart is not in control, then the control limits for the $image\xbar.gif$ chart will be inaccurate and may inappropriately signal an out of control condition on the $image\xbar.gif$ chart.

Example Output

TEST 1. One point more than 3.00 standard deviations from center line.

Test Failed at points: 8

TEST 5. 2 out of 3 points more than 2 standard deviations from center line (on one side of

CL).

Test Failed at points: 17

TEST 6. 4 out of 5 points more than 1 standard deviation from center line (on one side of

CL).

Test Failed at points: 12, 13, 19, 20

Interpretation

The camshaft data did not fail any of the tests for special causes. Therefore, the R chart is in control and it is appropriate to examine the $image\xbar.gif$ chart.