np Chart
The np chart is a SPC Chart PRO attribute chart that monitors the count of defective units per subgroup, when the number of units inspected is (near) constant.
The np chart is the p chart's sibling: both watch pass/fail data under the binomial distribution, but the np chart plots the raw count of defectives instead of the proportion. That is only meaningful when every subgroup inspects roughly the same number of units — with a constant sample size, a count of 7 defectives always means the same thing. 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, and the control limits are set three sigma out under the binomial distribution. 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.
A subgroup is flagged when its defective count falls outside the limits.
Setting It Up
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 count or a 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 flat limits, the classic np picture.
Methods and Formulas
Let each subgroup i contribute dᵢ defective units out of nᵢ inspected, and let n̄ be the average sample size.
| 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 flat; the per-point form above is what lets the chart stay correct if the sample size happens to vary.
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. For requirements, licensing, and support, see the SPC Chart PRO documentation.