Minitab conducts up to four tests for special causes for the Laney U' chart, which detect points beyond the control limits and specific patterns in the data. When a data point fails a test, Minitab reports it in the Session window and marks the point on the chart with a red symbol. A failed point indicates that there is an unusual value or a nonrandom pattern, either of which may be the result of special cause variation. These points should be investigated.

For the medication error data, subgroup 23 failed Test 1 because it is more than 3 s from the center line. This point is outside of the control limits, which suggests the presence of special cause variation. You should investigate this point to determine the nature of the problem.