p and np control charts
Monitors the proportion (p) or number (np) of nonconforming items in samples; limits widen for smaller samples.
Menu: Graphs → More charts → Control chart: p, np…
When should I use it?
- Tracking defective rates per lot, day or shift.
What are the assumptions?
- Each item is either conforming or not; samples of known size.
How do I run it in StatLab?
- Choose the count of nonconforming items and the sample size.
- Choose p or np.
How do I read the output?
| Output | Meaning |
|---|---|
| Center line | Total nonconforming / total inspected. |
| Limits | p̄ ± 3·√(p̄(1 − p̄)/n), floored at 0. |
What are common mistakes?
- Using np with very different sample sizes (use p).