AiVibeIndustry 4.0 & Manufacturing Operations
Attribute Control Charts (p, np, c and u Charts)
Attribute control charts monitor count data, such as the number or proportion of defective items or the number of defects per unit, to show whether a process producing pass or fail results or countable defects is stable over time.
Four charts are commonly used. The p chart plots the proportion of defective items in each sample and allows sample sizes to vary. The np chart plots the number of defective items when the sample size is constant. The c chart plots the number of defects in a constant inspection unit, such as flaws per roll of fabric, and the u chart plots defects per unit when the inspected quantity varies. The p and np charts are based on the binomial distribution, and the c and u charts on the Poisson distribution.
Attribute charts are used where characteristics are judged rather than measured, such as visual defects, leak test results, solder joint faults or customer complaints, and where automated inspection systems already record pass or fail outcomes. They are simple to maintain and give a broad picture of quality performance across a line or plant.
Attribute data carry less information than measurements, so these charts typically need much larger samples to detect a change, and they react only after defects occur. Where a characteristic can be measured, variables charts are usually more sensitive. When defect rates are very low, conventional attribute charts become ineffective and alternatives based on the time or number of units between events are used. Consistent defect definitions are essential.
Key points
- p and np charts track defective items; c and u charts track defects.
- p and np use the binomial distribution; c and u use the Poisson distribution.
- p and u charts handle varying sample sizes.
- Need larger samples than variables charts to detect the same change.
Where AiVibe comes in
AiVibe designs and manufactures the AiAmbA AI Factory as the OEM: edge devices plus AI agents that let people talk to PLC, CNC and robot controllers in plain language. Existing lines are retrofitted with edge devices, and a trained operator confirms every change an agent proposes.
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