Control charts (Shewhart charts) are the standard tool for monitoring the analytical performance of certified reference materials (CRMs) over time. Most laboratories plot CRM results against the certified value and apply ±2σ warning limits and ±3σ action limits.
But passing a ±2σ threshold is not sufficient evidence of good performance. Control charts must also be interpreted for systematic patterns — the Western Electric Rules define several conditions that indicate process instability even when no individual point exceeds the action limit:
– Rule 1 (Spike): One point beyond ±3σ → investigate contamination or calibration failure.
– Rule 2 (Trend): Eight consecutive points on the same side of the mean → suggests systematic drift, reagent degradation, or instrument baseline shift.
– Rule 3 (Stratification): Fifteen consecutive points within ±1σ → may indicate data rounding or insensitive measurement.
– Rule 4 (Mixture): Eight consecutive points alternating above and below the mean → suggests two alternating analytical populations (e.g., instrument recalibrated mid-batch).
In geochemical QA/QC, drift patterns are particularly significant. A gradual upward trend in a CRM over months can indicate reagent contamination building in the preparation workflow — invisible to single-batch review but detectable in time-series charting.
Effective control chart use requires: long-term data retention, trend analysis beyond single batches, and investigation protocols triggered by pattern detection — not only limit violations.
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