Proportion Chart (P-chart)

Introduction: P-chart

A P-chart, or proportion chart, is a type of control chart used in quality control to monitor the proportion of defective units in a process. Unlike a C-chart, which counts the total number of defects, the P-chart focuses on whether each unit is defective or not, making it ideal for processes with pass/fail or yes/no outcomes.

Background

The P-chart is part of the control chart family introduced by Walter A. Shewhart in the early 20th century. It is widely applied in industries where monitoring the percentage of defective units provides valuable insights into process stability and capability. The chart is particularly useful when sample sizes vary, as it adjusts control limits accordingly.

Key Elements / Features

• Focus on Defective Units: Tracks the proportion of items that fail inspection in each sample.
• Variable Sample Sizes: Control limits are recalculated based on sample size, allowing flexibility.
• Control Limits: Upper and lower control limits identify unusual variation or instability.
• Binary Classification: Each unit is classified as either defective or non-defective.
• Process Monitoring: Evaluates whether the process remains stable over time.

Applications / Examples

• Manufacturing: Monitoring the proportion of defective items in production batches.
• Healthcare: Tracking the percentage of errors in prescriptions or patient records.
• Service Industries: Measuring rejected transactions or incorrect data entries.
• Education: Analysing the proportion of failed assessments or errors in marking.

Relevance / Impact

The P-chart supports organisations by:

• Detecting deviations from expected quality levels.
• Providing an early warning system for process instability.
• Reducing risk by identifying when intervention is required.
• Supporting continuous improvement and regulatory compliance.

By focusing on proportions rather than counts, the P-chart offers a powerful way to monitor quality when processes are evaluated on a pass/fail basis.

See also

• C-chart
• Statistical Process Control (SPC)
• Control Limits
• Quality Assurance