What is center line drift on a QC control chart?

• A. Shift in the target mean over time.
• B. Random fluctuations around the mean.
• C. Inaccurate time axis.
• D. Calibration at the start only.

Answer: (A)

Center line drift occurs when the average value the chart is tracking moves as data accumulate. The center line represents the process mean or target. If that mean gradually shifts up or down over time, the center line itself appears to drift. This signals a systematic change in the process rather than random variation around a fixed target. Causes might include instrument drift, calibration changes, or evolving process conditions that bias measurements over time. It differs from random fluctuations, which stay centered around a fixed mean, and it’s not about the time axis or a one-time calibration—it's the mean itself that is changing.