How do you interpret a quality control chart and identify when a test run is out of control?

• A. Look for points outside control limits, non-random patterns, trends, or shifts; investigate potential causes
• B. Look for points inside control limits only
• C. Ignore all results
• D. Average all results and accept

Answer: (A)

Interpreting a quality control chart is about distinguishing normal, random variation from signals that the process has changed. The chart’s center line represents the typical average, and the upper and lower control limits mark the range we’d expect if the process is in control.

A test run is out of control when you see signs of special-cause variation: a point that falls outside the control limits, or non-random patterns such as a sequence of points trending up or down, a run on one side of the center line, or a sudden shift in the mean or in variability. These signals point to something in the process changing—perhaps equipment, reagents, method, personnel, or environment—and they should prompt an investigation into possible causes and corrective actions, followed by continued monitoring to confirm the process has returned to stability.

Points that stay inside the limits aren’t an automatic guarantee of control, and simply calculating an average or ignoring results would miss real signals.