Control Chart
Definition
Time-series plot of a QC measurement with warning limits at ±2σ and action limits at ±3σ from validation data. Patterns such as seven consecutive points on one side trigger investigation before drift reaches case work.
- Warning limit
- ±2σ from validation mean
- Action limit
- ±3σ from validation mean
- Trend rule
- Seven consecutive points on one side
Common questions
What should happen when a control chart shows an action-limit exceedance?+
Casework analysis is typically halted until the root cause is investigated, and the instrument or method is reverified as in control before further samples are run.
Why does a run of points within limits still matter if none exceed ±3σ?+
A sustained shift to one side signals a systematic drift, such as reagent degradation, that could eventually bias results even though no single point has yet crossed the hard action threshold.
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