What is the primary purpose of a control chart?

• A. To document customer specifications.
• B. To monitor process stability and detect assignable-cause variation so the process remains in statistical control.
• C. To estimate process capability using Cp and Cpk.
• D. To count defects in finished products.

Answer: (B)

Control charts monitor how a process performs over time and keep it in statistical control by separating natural, everyday variation from special causes. They plot measurements with a center line (the average) and control limits (typically set at ±3 standard deviations). When the process is stable, data points fluctuate randomly within the limits. If a point falls outside the limits or shows a non-random pattern, that signals an assignable cause and prompts investigation and corrective action. The goal is to detect problems early, maintain predictability, and reduce variation.

Documenting customer specifications focuses on product requirements, not the process behavior. Estimating process capability with Cp and Cpk relies on a stable process, which control charts help establish, but they’re not the primary purpose themselves. Counting defects in finished products is inspection-based, not monitoring the process over time to detect variation.