np Chart
The np chart is an attribute chart in SPC Chart PRO that monitors the count of defective units per subgroup when the number of units inspected is constant or nearly constant.
Like the p chart, the np chart monitors pass/fail data using the binomial distribution. It plots the count of defective units. Counts are directly comparable when each subgroup contains the same number of inspected units. If your sample size varies from subgroup to subgroup, use the p chart instead, which normalizes to a proportion.
Select np chart (count defective) in the Chart Type dropdown.
How It Works
Each row of your query is one subgroup. The value field holds the count of defective units in that subgroup, and a second column (or a fixed option) gives the number of units inspected. The panel plots the defective count directly.
The center line is the average defective count. The control limits use the binomial model, with each subgroup's limits centered on its expected count nᵢ·p̄. When the sample size is constant the limits are flat; if the sample size does drift, the limits step to follow it while the center line stays at the average count.
Enable Nelson run rules to flag patterns in the defective counts.
Setting It Up
The defective count must not exceed the number of units inspected.
In the SPC section, with the np chart type selected:
| Option | Description |
|---|---|
| Sample size field | A numeric field in your query giving the number of units inspected per subgroup. It is hidden from the plot automatically. |
| Fixed sample size | A constant number inspected per subgroup, used when no sample-size field is selected. For an np chart this is the common case — one number for the whole chart. |
Rows with a missing or negative count, or a missing or non-positive sample size, are treated as gaps: they are not plotted and do not contribute to the center line or the limits.
The np chart is designed for a constant sample size. If you already know every subgroup inspects the same number of units, set Fixed sample size to that number and leave the sample-size field empty — the chart then draws constant limits.
Methods and Formulas
Let each subgroup i contribute dᵢ defective units out of nᵢ inspected, and let n̄ be the average sample size.
| Quantity | Formula |
|---|---|
| Plotted value | dᵢ |
| Center line | n̄·p̄, where p̄ = Σdᵢ ⁄ Σnᵢ |
| Control limits | nᵢ·p̄ ± 3·√( nᵢ·p̄(1−p̄) ) |
The lower limit is floored at 0 — a defective count can never be negative. With a constant sample size, nᵢ = n̄ and the limits are constant. The formula above also allows limits to vary with sample size, but a p chart is recommended for comparing subgroups of different sizes.
Choosing the np Chart
| Scenario | Chart |
|---|---|
| Pass/fail inspection, constant sample size | np |
| Pass/fail inspection, sample size varies | p |
| Large subgroups and limits that look too tight | Laney p′ |
The sample-size configuration and chart type are saved with the dashboard. Like every chart type, the np chart works with both a time and a numeric X-axis.
The np chart is part of SPC Chart PRO. See requirements, licensing, and support.