A P chart (proportion chart) is a type of control chart in quality control that monitors the proportion of defective items or nonconforming units in a process over time. It is designed for use with binomial data, where each item is either defective or not. P charts are especially useful when the sample size is consistent across time periods. The purpose of the chart is to detect whether the process has gone out of control, meaning that the proportion of defects is no longer consistent with expected random variation. An out of control process can signal a change in materials, methods, equipment, or human error that requires investigation and correction.
The center line of a P chart represents the average proportion defective (p̄) across all samples. The upper and lower control limits (UCL and LCL) are typically set at 3 standard errors above and below the center line. These limits help determine whether variation in the process is due to common causes (expected) or special causes (unexpected and potentially problematic).
Suppose a manufacturer inspects 100 items from each of 20 production batches and records the proportion of defective items in each batch. The P chart below shows those proportions over time, with control limits based on the average defect rate.
The chart displays the proportion of defective items in each batch as yellow dots. The green dashed line represents the average defect rate, while the red dashed lines mark the upper and lower control limits. All points fall within the control limits, and no unusual patterns appear, which suggests that the process is in statistical control. The observed variation is consistent with random fluctuations expected in a stable process.