# Measurement uncertainty

> Measurement uncertainty is the quantified range around a lab result within which the true value is expected to lie, typically stated as ± a value at about 95% confidence.

- Canonical: https://genetixbay.com/knowledge/glossary/measurement-uncertainty.html
- Also written: uncertainty of measurement, MU, measurement error, ± uncertainty, expanded uncertainty, error bars
- Glossary category: Lab testing · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Every measurement carries uncertainty from weighing, extraction, calibration, instrument response, the analyst and, above all, [sampling](https://genetixbay.com/knowledge/glossary/homogenisation.md). Laboratories accredited to [ISO/IEC 17025](https://genetixbay.com/knowledge/glossary/iso-17025.md) must estimate it. It is commonly reported as an expanded uncertainty (coverage factor k = 2, roughly 95% confidence), for example "total [THC](https://genetixbay.com/knowledge/glossary/thc.md) 21.0% ± 1.5% w/w".

## In practice

That ±1.5% means a reading of 21.0% is consistent with a true value anywhere from about 19.5% to 22.5% for that sample. Two COAs of 20.1% and 21.7% from the same batch may not disagree at all. Uncertainty usually grows with heterogeneous samples: a single bud from the top of a plant can differ by several points from a lower one, which the lab's own uncertainty doesn't capture.

## Units and formula

Same units as the result; also expressed as relative %. Expanded uncertainty U = k × combined standard uncertainty, with k = 2 for about 95% coverage.

## Don't confuse with

- **LOD/LOQ**: thresholds at the low end, not the spread of a result.
- **Lab-to-lab bias**: a systematic offset, which uncertainty estimates may not include.

## Myth check

Uncertainty explains small differences, not 10-point gaps. See [buyer beware](https://genetixbay.com/knowledge/basics/buyer-beware-label-claims.md).

## Related terms

[Potency testing](https://genetixbay.com/knowledge/glossary/potency-testing.md), [Homogenisation (sampling)](https://genetixbay.com/knowledge/glossary/homogenisation.md), [Proficiency testing](https://genetixbay.com/knowledge/glossary/proficiency-testing.md), [ISO/IEC 17025](https://genetixbay.com/knowledge/glossary/iso-17025.md), [Label inflation](https://genetixbay.com/knowledge/glossary/label-inflation.md)

## Read more

- [Buyer beware: potency labels, strain names and other claims that don't hold up](https://genetixbay.com/knowledge/basics/buyer-beware-label-claims.md)

## Sources

1. Ellison SLR, Williams A (eds). 2012. Eurachem/CITAC Guide: Quantifying Uncertainty in Analytical Measurement, 3rd edition.
2. ISO/IEC 17025:2017. General requirements for the competence of testing and calibration laboratories. International Organization for Standardization, Geneva.