The P chart plots the proportion of nonconforming units (also called defectives). For example, you can use a P chart to monitor the following:
- The proportion of flights that depart late
- The proportion of bicycle tires that are flat
- The proportion 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.
The P chart is the most widely used attribute control chart.
A delivery service manager uses a P 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, 8% of the delivery vehicles are out of service on any particular day. The proportion 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 rate of defectives.