NP Control Chart: Nonconforming Count at Fixed Sample Size
What is an NP Chart?
The NP chart monitors the count of nonconforming units in each inspected batch and, like the P chart, is based on the binomial distribution - but it plots counts rather than rates. When the sample size is fixed, the NP chart limits form a straight line, making it more intuitive and easier to explain to shop-floor staff.
When to Use It
Use it when every batch is inspected at a fixed quantity - for example always sampling 200 pieces or 5 boxes per lot. If inspection quantities vary considerably across batches, switch to the P chart, whose limits adjust with the sample size.
How to Read It
A point above the upper limit means that batch's nonconforming count is significantly higher than normal - investigate the special cause immediately. A point below the lower limit (when present) indicates an unusual quality improvement that should also be confirmed (e.g., not due to missed inspection or misclassification). The tool flags signals automatically.
Control Limits
The center line is the average nonconforming count npbar (the mean of batch counts); the limits are npbar +/- 3*sqrt(npbar*(1-pbar)), where pbar is the average nonconforming rate. The tool performs the calculation and plots the chart.