The NP chart plots the number of nonconforming units (also called defectives). For example, you can use an NP chart to monitor the following:
- The number of flights that depart late
- The number of bicycle tires that are flat
- The number of printed logos that are smudged
While a unit may have many quality characteristics that can be evaluated, it is always considered as either conforming or nonconforming.
A delivery service manager uses an NP chart to monitor the proportion of delivery vehicles that are out of service each day for 2 months. A vehicle that is out of service is considered a defective unit.
On average, 100 of the delivery vehicles are out of service on any particular day. The number of defective units for day 19 is out of control. The manager should try to identify any special causes that may have contributed to the unusually high number of defectives.