# The 2SD/3SD rule

> multiple 2SD OR one 3SD fails; a single 2SD warns

Source: https://geodb.io/guide/topic/qaqc_crm_2sd_3sd_rule/ · Category: QAQC
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:

1. **One result beyond 3SD** — outright rejection (random odds ~0.3%).
2. **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.