c Chart
The c chart is an attribute chart in SPC Chart PRO that monitors the number of defects found in a constant inspection area — a fixed unit of product, area, time, or volume.
The distinction from a p or np chart matters: a p/np chart counts defective units (each unit is pass or fail), while a c chart counts defects — a single unit can have several defects. Examples include blemishes on a painted panel, weld flaws per assembly, or errors per printed page. As long as the inspection area is the same every time (one panel, one assembly, one page), the count is directly comparable and no sample size is needed.
Select c chart (defect count) in the Chart Type dropdown.
How It Works
Each row of your query is one inspection unit, and the value field holds the defect count for that unit. Because the inspection area is fixed, the c chart needs no sample-size column — it is the one attribute chart with no sample-size option.
The center line is the average defect count, and the control limits are set at three standard deviations from the center line under the Poisson distribution, which models defect counts over a fixed inspection area. The limits are constant within each stage.
Enable Nelson run rules to flag patterns in the defect counts.
Setting It Up
Select the c chart as the Chart Type and configure your query to return defect counts, with one row per inspection unit. No sample-size field is required.
Rows with a missing or negative count are treated as gaps: they are not plotted and do not contribute to the limits.
The c chart assumes the inspection area is constant — the same panel, page, or time window every time. If the size of the inspected area varies (different-sized panels, different production volumes), the counts are no longer directly comparable; use the u chart instead, which normalizes defects to a per-unit rate.
Methods and Formulas
Let each unit i contribute cᵢ defects.
| Quantity | Formula |
|---|---|
| Plotted value | cᵢ |
| Center line | c̄ = mean of cᵢ |
| Control limits | c̄ ± 3·√c̄ |
The lower limit is floored at 0. Under the Poisson distribution the variance equals the mean, so the single quantity c̄ sets both the center line and the spread — which is why the limits are constant and no sample size enters.
Choosing the c Chart
| Scenario | Chart |
|---|---|
| Counting defects in a fixed inspection area | c |
| Counting defects, inspected area varies | u |
| Counting defective units (pass/fail) | p / np |
The chart type is saved with the dashboard. Like every chart type, the c chart works with both a time and a numeric X-axis.
The c chart is part of SPC Chart PRO. See requirements, licensing, and support.