Each test for special causes detects a specific pattern in the data plotted on the U chart. When a data point fails a test, Minitab reports it in the Session window and marks it on the U 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.

The transcription 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 test:

·    Samples 6 and 18.failed Test 1, which 1 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 rate of defects per unit of measurement across samples.

These results indicate that the process is unstable and that the factors causing this variation should be identified and corrected.