C charts assess whether the number of defects in each sample 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 C 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 wallpaper data failed the tests for special causes, which provides evidence that the process is out of control due to the presence of special-cause variation. Two data points failed the following tests:

·    Samples 12 and 13 failed Test 1, which looks for points that are more than 3 s from the center line. This test provides the strongest evidence for lack-of-control in a process and detects shifts in the distribution of defects.

These results indicate that the process is out of control in regards to the number of defects. The factors causing this variation should be identified and corrected.