# Expected failure rate

> ~4.6% outside 2SD is normal; zero is suspicious

Source: https://geodb.io/guide/topic/qaqc_expected_failure_rate/ · Category: QAQC
The most counter-intuitive fact in QC: **a healthy program is supposed to
show failures.** Under normality, ~4.6% of CRM results fall outside ±2SD
**by chance alone**, and ~0.3% outside ±3SD. Snowden Optiro states it
directly: *"in excess of 4% of all results should legitimately be outside
[2SD]."*

So a program showing **near-zero 2SD exceedances is suspicious, not
excellent** — it usually means one of two things:

- the certified SDs you're checking against are **too wide** (a real,
  documented problem — an over-wide SD makes the whole program blind), or
- the lab has **identified your standards** and is treating them specially.

AMIS's practical benchmark: a good laboratory reports within 2SD with a
failure rate **under 10%** — under, not at zero.

## The reporting rule that follows

**Never report a failure rate without its threshold and the expected
rate.** "5% of standards failed" is *normal* at a 2SD threshold and a 16×
excess at 3SD — the same number, opposite conclusions. Rainy River's QP
used "<5% failure rate acceptable per CRM" — explicitly at 3SD.

## In geoDB

The QC pages report pass rates per category (standards, blanks,
duplicates) against the thresholds described in the other topics. geoDB
does not currently annotate rates with the statistically expected rate —
when you read "96% passing", remember 95.4% is the theoretical
expectation at 2SD.