What does a p-chart specifically monitor in a process?

A p-chart, or proportion chart, is specifically designed to monitor the proportion of nonconforming units in a process over time. This type of chart is part of statistical process control (SPC) and helps track the fraction of items in a sample that do not meet quality standards or specifications.

By focusing on the proportions of nonconforming units, a p-chart allows organizations to assess the stability of a process and whether it is in control. It is especially useful when dealing with quality attributes that are categorical—such as pass/fail or conforming/nonconforming classifications. The chart visualizes these proportions, providing insights into trends and variations in quality that may arise due to various factors within the production process.

Other options, such as monitoring weight variation, weight and length, or quantities of materials used, pertain to different types of data and charts, such as x-bar charts for continuous data, not specifically the p-chart's focus on proportions of nonconformance.