The NP chart plots the number of defective items (also called nonconforming units). The center line represents the average number of defectives. 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 number of defectives.
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. For more information, go to Specify subgroups to estimate parameters for NP Chart.
The chart shows that, on average, 25 of the delivery vehicles are out of service each day. The number of defective units for day 19 is out of control. The manager should identify any special causes that contribute to the unusually high number of defectives.
When you hold the pointer over a red point, you can get more information about the subgroup.