P Chart

The P chart is one of the most commonly used attribute control charts. It monitors the proportion of defective units in a sample and adjusts for varying sample sizes. This makes it ideal for processes where the number of inspected units changes from one period to the next. 

Each point on a P chart represents the percentage of units that were defective in a given sample. The control limits are calculated based on the average defect proportion and adjusted for sample size. Larger samples produce narrower limits; smaller samples produce wider limits. This ensures that the chart accurately reflects expected variation. 

The P chart is useful in a wide range of environments—manufacturing, service, healthcare, and administrative processes. It provides a clear, intuitive view of quality performance and helps teams detect shifts, spikes, or trends in defect rates. 

Interpreting a P chart requires attention to both statistical and practical significance. A point above the upper control limit indicates a special cause that requires investigation. A point below the lower control limit may indicate improvement—but only if the process is stable and the change is sustained. 

In the Control phase, the P chart provides a reliable, flexible way to monitor defect proportions, maintain stability, and sustain improvements over time.