Consecutive outliers

A single outlier is noise; a pattern of them is bias. Trending rules formalize this, and OREAS endorses them by name on its certificates: "Westgard Rules extend the basics of single-rule QC monitoring."

The core set:

Rule	Meaning
1-3s	one result beyond ±3SD → reject
2-2s	two consecutive beyond ±2SD, same side → systematic error
R-4s	range between two controls exceeds 4SD
4-1s	four consecutive beyond ±1SD, same side
10-x	ten consecutive on the same side of the mean

The mining-specific one is 2-2s — Bishi (after Barry Smee): results rejected when "two consecutive batches show two CRM results beyond the two-standard-deviation limit on the same side of the mean." Under normality that pattern occurs randomly only 2.3% of the time, which is exactly why it detects bias rather than punishing noise.

Whether the long-run trigger is 7, 9 or 10 consecutive same-side results depends on the rule-set you adopt (Westgard vs Nelson) — a genuine convention choice. Full multi-rule stacks are not standard in mining; 2-2s is the one with mining-specific backing.

In geoDB

geoDB does not evaluate trending rules — every verdict is per-result. The control charts on the QC pages plot results in sequence against the ±2SD/±3SD bands, which makes same-side runs easy to spot by eye; for a formal check, ask Petra to run one over your data.