The 2SD/3SD rule
The standard CRM acceptance rule — printed verbatim on OREAS certificates — is asymmetric, and most people misremember it:
"warning or rejection for multiple 2SD outliers, or rejection for individual 3SD outliers… their precise application should be at the discretion of the QC manager concerned."
A single result between 2SD and 3SD is a warning, not a failure. Under normality ~4.6% of results land outside 2SD by pure chance — failing each one would mean rejecting good batches constantly. Failure comes two ways:
- One result beyond 3SD — outright rejection (random odds ~0.3%).
- Multiple 2SD outliers — especially on the same side of the mean, which signals systematic bias rather than noise.
Note that the "multiple outliers" half is a pattern across results, not a property of any single result — a per-sample verdict can never express it. That is what trending rules are for (see the consecutive-outlier topic).
Some practitioners do treat ±2SD itself as failure, and Geostats recommends 3SD as the only gate — both are defensible conventions. Know which one your project uses.
In geoDB
geoDB judges each standard individually: within 2σ passes, 2–3σ shows a warning, beyond 3σ fails — exactly the individual half of the rule. The "multiple 2SD outliers" pattern half is not evaluated automatically; use the control charts on the QC pages to spot it by eye, or ask Petra for a trend analysis.