p-Chart (Defect Rate)
The p-chart monitors the defect rate per batch (p = defects / sample size). For attribute data (pass/fail). Sample sizes may vary, so control limits adjust per batch.
CSV format: defects column + sample_size column
In a PCB SMT process, 200 boards are sampled per shift and solder defects recorded. The p-chart visualizes defect rate so QC staff can immediately spot solder degradation, paste printing offset, or placement accuracy drift. Compared with 100% visual inspection, the p-chart gives statistically meaningful trend interpretation and auto-computed control limits as action triggers.
Defects in three batches: 8, 12, 5; each batch n = 200 p̄ = (8+12+5) / (200×3) = 25/600 = 0.0417 (4.17%) UCL = p̄ + 3×√(p̄×(1-p̄)/n) = 0.0417 + 3×√(0.0417×0.9583/200) = 0.0417 + 0.0420 = 0.0837 (8.37%) LCL = max(0, 0.0417 - 0.0420) = 0 → Batch 2 (6% defect rate) is near the UCL; investigate the cause
FAQ
Related Tools
c-Chart (Defect Count)
p-chart monitors defect rate; c-chart monitors defects per unit. Use c-chart when one unit can have multiple defects.
Open toolControl Chart (X̄-R)
p-chart suits attribute data; X̄-R suits variable data (e.g. dimensions, weight). Confirm your data type before choosing a chart.
Open toolAQL Acceptance Sampling
p-chart provides continuous in-process monitoring; AQL sampling plans handle incoming/outgoing lot acceptance. They complement each other.
Open tool