What is the primary purpose of a control chart in statistical process control?

• A. To identify process capability relative to specifications.
• B. To detect when a process is out of statistical control.
• C. To set process target values.
• D. To compute the overall process yield.

Answer: (B)

Monitoring a process over time with a control chart is about distinguishing normal, random variation from unusual, assignable variation. The primary purpose is to detect when a process is out of statistical control, signaling that special causes may be affecting performance and that investigation is needed. Control charts plot a statistic (like the mean or range) with control limits so signals outside those limits or nonrandom patterns indicate potential problems that should be addressed to bring the process back into control. This helps prevent defects by catching issues early. It’s not meant for assessing how a process measures up to specifications (that’s capability analysis), setting target values, or calculating overall yield.