The U chart plots the number of defects (also called nonconformities) per unit. It is possible for an item to have one or more defects but still be acceptable in function and performance. The center line represents the mean number of defects per unit (or subgroup). The control limits, which are set at a distance of 3 standard deviations above and below the center line, show the amount of variation that is expected in the defect rate.
Red points indicate subgroups that fail at least one of the tests for special causes and are not in control. If the same point fails multiple tests, then the point is labeled with the lowest test number to avoid cluttering the graph. If the chart shows out-of-control points, investigate those points.
Out-of-control points can influence the estimates of process parameters and prevent control limits from truly representing your process. If out-of-control points are due to special causes, then consider omitting these points from the calculations.
In these results, the average defect rate is approximately 0.32. The process does not appear to be in control because the final subgroup fails at least one test for special causes. When you hold the pointer over a red point, you can get more information about the subgroup. To determine which tests each point fails, review the output in the Session window.