C Chart (Count of Defects)
Monitor the number of defects when the inspection area or unit size is constant. The C chart counts total defects per unit, whether or not each unit is defective, using the Poisson distribution to set its control limits.
Create C Chart →What is a C Chart?
A C chart is an attribute control chart that monitors the total number of defects per inspection unit when the inspection area or opportunity is constant. Unlike the P and NP charts, which count defective units, the C chart counts defects, the individual nonconformities, and a single unit may contribute several.
It requires a constant inspection unit, a fixed area, length, or opportunity in which defects can occur, such as a fixed panel size, a set length of wire, or a standard batch. Because the area of opportunity is the same each time, the control limits stay constant across the chart.
The C chart is based on the Poisson distribution, which models the count of rare, independent events in a fixed interval, exactly the situation of counting defects in a constant area. It answers whether the defect count is in statistical control, and it is the right choice when the number of flaws, not the number of bad units, is what matters.
In plain terms: When you count flaws (not bad units) in a fixed area, scratches on a standard panel, errors per standard form, the C chart tracks the total defect count over time. One item can have several defects. Because the inspection area is always the same size, the control limits are flat.
Key Points
Count of Defects
Counts total defects per inspection unit, not defective units. One unit can contribute several defects.
Constant Inspection Unit
Requires a fixed area or opportunity for defects, so the control limits stay constant.
Poisson Basis
Based on the Poisson distribution, which models counts of rare independent events in a fixed interval.
Key Formulas
Reading the Chart
With a constant inspection unit the limits are flat. A point beyond the limits, or a non-random pattern, signals a special cause changing the defect count. Because the lower limit can be negative, it is set to zero when the calculation falls below it.
The C chart counts defects, not defectives. This is the crucial distinction from P and NP charts: here one item can add several to the count. If the inspection area varies between samples, the C chart's constant-area assumption is violated and a U chart, which uses defects per unit, is required.
Assumptions & Validation
Constant Inspection Unit
The area or opportunity for defects is the same each time.
If violated: If the inspection area varies, use a U chart (defects per unit).
Counting Defects
Individual defects are counted, and a unit may have several.
If violated: To classify units as defective or not, use a P or NP chart.
Independent, Rare Defects
Defects occur independently, matching the Poisson model.
If violated: Address clustering or dependence between defects.
⚠️ Check assumptions first
The C chart counts defects, not defective units, and requires a constant area of opportunity. Two conditions must both hold: you are counting flaws (a unit can have several), and the inspection unit is a fixed size each time. If the inspection area varies, the constant-limit assumption breaks and you must use a U chart. If you are classifying whole units as pass or fail, a P or NP chart is correct instead.
When NOT to Use C Chart
Variable Inspection Area
When the inspection area or opportunity changes between samples, use a U chart.
Counting Defectives
To classify units as defective or not, use a P chart (variable size) or NP chart (constant size).
Continuous Data
For measured variable data, use an X-bar R, X-bar S or XmR chart.
Industry Applications
Surface Defects
Count blemishes, scratches or flaws on a standard-size panel or sheet.
Fixed-Unit Inspection
Count defects per standard form, assembly or fixed length of material.
Process Control
Detect shifts in the defect count of a stable process.
Error Monitoring
Track the number of errors per fixed-size document or transaction batch.
Frequently Asked Questions
What is a C chart used for?
A C chart monitors the total number of defects per inspection unit when the inspection area or opportunity is constant. It counts defects, the individual nonconformities, rather than defective units, so a single item can contribute several to the count. Based on the Poisson distribution, it determines whether the defect count is in statistical control and is used when the number of flaws, not the number of bad units, matters.
What is the difference between a C chart and a P chart?
A C chart counts defects, individual nonconformities, in a constant inspection area and uses the Poisson distribution. A P chart counts defective units, classifying each unit as pass or fail, and uses the binomial distribution while handling variable sample sizes. The core difference is defects versus defectives: the C chart counts flaws, which may be several per item, while the P chart counts bad units.
When should I use a C chart instead of a U chart?
Use a C chart when the inspection unit is a constant size or opportunity each time, so the control limits stay flat. Use a U chart when the inspection area varies between samples, because it plots defects per unit and adjusts the limits accordingly. Both count defects using the Poisson distribution; the choice depends on whether the area of opportunity is constant or variable.
What distribution is the C chart based on?
The C chart is based on the Poisson distribution, which models the number of rare, independent events occurring in a fixed interval of time, space or opportunity. Counting defects in a constant inspection area fits this model well. The Poisson basis is why the control limits are calculated as the average count plus or minus three times its square root, and why defects should occur independently.
Why is the lower control limit sometimes zero on a C chart?
The formula for the lower control limit subtracts three times the square root of the average count from that average, which can produce a negative number when the average defect count is low. Since a count of defects cannot be negative, the lower control limit is set to zero in that case. This simply means that no realistically low count would signal an out-of-control condition on the low side.
Can one unit have multiple defects on a C chart?
Yes, and this is central to what the C chart does. It counts defects rather than defective units, so a single inspected item can contribute several defects to the total for that inspection unit. This is the key distinction from P and NP charts, which classify each whole unit as defective or not. When multiple flaws per item matter, a C or U chart is the appropriate tool.
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