What is the primary purpose of a control chart in a quality improvement effort?

• A. To present raw data without interpretation.
• B. To forecast demand for staffing for the next quarter.
• C. To distinguish common-cause from special-cause variation and detect signals of a process change.
• D. To calculate the overall budget impact of changes.

Answer: (C)

The main idea is to monitor a process over time and tell apart random, inherent variation from variation caused by a specific change. A control chart does this by plotting data in time order with a centerline (the typical value) and upper and lower control limits that reflect expected common-cause variation. If the process stays within those limits in a random pattern, it’s considered in control. When a point falls outside the limits or you see nonrandom patterns (like a systematic trend or a run of points), that signals special-cause variation and suggests a possible process change that may need investigation. This approach guides improvement by showing when the system has changed and whether it’s stable enough to assess the impact of further changes. It’s not just about presenting raw data, forecasting staffing needs, or calculating budget impact.