np Chart is a control chart for monitoring the count of defective units in constant-size samples, where each unit is classified as either defective or acceptable.
What Is an np Chart?
- Metric: Number of defective units (np) per sample
- Requirement: Constant sample size (n) across all samples
- Distribution: Binomial distribution assumption
- Related: p-chart tracks proportion defective (variable sample size)
Why np Charts Matter
For attribute data with pass/fail inspection of fixed sample sizes, np charts provide simpler arithmetic than proportion charts while monitoring process stability.
np Chart Example:
Sample size: n = 50 units per lot
Average defective rate: p̄ = 0.04
Center Line: np̄ = 50 × 0.04 = 2.0 defectives
UCL = np̄ + 3√(np̄(1-p̄)) = 2 + 3√(2×0.96) = 6.2
LCL = np̄ - 3√(np̄(1-p̄)) = 2 - 4.2 = 0 (use 0, not negative)
When to Use np vs. p Chart:
| Condition | Chart |
|---|---|
| Fixed sample size | np chart |
| Variable sample size | p chart |
| Count defects per unit | c or u chart |
np chartdefective countattribute control chart
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