What is the purpose of a control chart in QC?

• A. To monitor a measured quantity over time and detect shifts or trends
• B. To replace calibration procedures
• C. To describe the assay's cost
• D. To record patient outcomes

Answer: (A)

A control chart monitors a measured quantity over time and is used to detect shifts or trends in the process. It plots each measurement in chronological order and compares it to a central value (the process mean) with upper and lower control limits that define expected variation. The goal is to distinguish normal, random variation from changes that indicate a problem. When all points stay within the limits and the pattern looks random, the process is considered in control. If a point falls outside the limits or a nonrandom pattern appears—such as a persistent drift, a sequence of points on one side, or a sudden shift—that signals a potential issue that should be investigated and addressed. This proactive monitoring helps maintain consistent quality, unlike calibration which establishes measurement accuracy at a single point in time, and it doesn’t describe costs or patient outcomes.