Which statement describes the behavior of a cusum when there is a systematic error?

• A. Cusum, in control
• B. Cusum, systematic error
• C. Westgard Rules
• D. 1(3s) Westgard Rules

Answer: (B)

A cusum chart sums up deviations from the target over time, so it is highly sensitive to consistent bias. When there is a systematic error, the deviations have the same sign most of the time, and the cumulative sum drifts in that direction. This creates a clear, steady trend and eventually crosses the control limits, signaling an out-of-control condition due to the bias. That’s why describing the situation as a cusum with systematic error is the best match.

A cusum that stays near zero would imply no persistent bias and stable performance, which isn’t the case here. Westgard rules refer to different, non-cusum QC criteria, and the 1(3s) rule is a Shewhart criterion, not a description of how cusum behaves under bias.