I-MR Chart

Test Results - MR Chart

The Moving Range (MR) chart assesses whether the process variation is in control. The tests for special causes detect points beyond the control limits and specific patterns in the data.

· When an observation fails a test, Minitab reports it in the Session window and marks it on the MR 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 observations failed the tests for special causes.

The MR chart must be in control before you can interpret the Individuals (I) chart. If the MR chart is not in control, then the control limits for the I chart will be inaccurate and may inappropriately signal an out-of-control condition.

Example Output

Test Results for I Chart

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: 20, 21

Interpretation

The liquid detergent data did not fail the tests for special causes on the MR chart. Therefore, the process is in control and it is appropriate to examine the I Chart.