Use an Individuals (I) chart to assess whether the process center is in control when your data are individual observations. 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 I 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 process variation must be in control before you can interpret the I chart. If the Moving Range (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.

For the liquid detergent data, the MR chart is in control so it is appropriate to examine the MR chart. Three observations failed two tests:

·    Observation 8 failed Test 1, which tests for single points that are more than 3 s from the center line. Test 1 provides the strongest evidence that a process is out of control.

·    Observations 20 and 21 failed Test 5, which tests for a run of two out of three points, with the same sign, more than two s from the center line. Test 5 provides additional sensitivity for detecting smaller shifts in process mean.

These test results indicate that the process average is unstable and the process is out of control, possibly due to the presence of special causes. You should try to identify and correct the factors causing the variation.