P Chart

P chart is the attributes control chart used to monitor the proportion of nonconforming units in samples over time - it is the standard SPC method for defectives-rate surveillance when sample size may vary.

What Is P chart?

• Definition: Chart of defective fraction calculated as defectives divided by total inspected units per sample.
• Data Type: Binary classification per unit, such as conforming versus nonconforming wafer.
• Sample Flexibility: Supports variable sample sizes with corresponding dynamic control-limit adjustment.
• Statistical Basis: Uses binomial-process assumptions for centerline and limit estimation.

Why P chart Matters

• Quality Visibility: Provides direct trend view of defectives rate at line and tool levels.
• Containment Speed: Rising defective fraction can trigger fast quality intervention.
• Operational Scalability: Practical for high-volume inspection streams with simple classification outputs.
• Compliance Support: Creates auditable record of quality-rate stability over time.
• Complementary Analytics: Works alongside defect-count charts for fuller quality insight.

How It Is Used in Practice

• Classification Discipline: Maintain consistent criteria for defective disposition across inspectors and shifts.
• Limit Maintenance: Recompute limits when baseline performance or sampling plan changes materially.
• Response Workflow: Connect P-chart signals to hold, review, and corrective-action procedures.

P chart is a foundational SPC chart for nonconformance-rate control - robust defective-fraction monitoring is critical for yield governance and early quality-risk detection.