Definition of Uncertainty Budgets in ISO/IEC 17025

In the world of testing and calibration laboratories, few concepts are as critical — yet sometimes misunderstood — as measurement uncertainty. ISO/IEC 17025:2017, the international standard specifying general requirements for the competence of testing and calibration laboratories, places strong emphasis on evaluating and reporting measurement uncertainty to ensure reliable, traceable, and comparable results.

Central to this process is the uncertainty budget — a structured tool that quantifies and documents all significant sources of uncertainty in a measurement. This article defines what an uncertainty budget is, explains its role in ISO/IEC 17025 compliance, and outlines best practices for laboratories.

What Is Measurement Uncertainty?

Measurement uncertainty is defined (per the International Vocabulary of Metrology – VIM) as a non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used.

In simpler terms: every measurement has some doubt attached to it. Uncertainty expresses how much the reported value might differ from the "true" value, typically at a specified level of confidence (e.g., approximately 95% with a coverage factor k=2).

Uncertainty differs from error: error is the difference between a measured value and the true value (often correctable), while uncertainty quantifies the remaining doubt after corrections.

The Role of Uncertainty Budgets in ISO/IEC 17025:2017

Clause 7.6 of ISO/IEC 17025:2017 explicitly requires laboratories to evaluate measurement uncertainty:

• 7.6.1: Laboratories shall identify the contributions to measurement uncertainty. All significant contributions — including those from sampling — must be considered using appropriate methods.
• 7.6.2: Calibration laboratories (including in-house calibrations) must evaluate uncertainty for all calibrations.
• 7.6.3: Testing laboratories must evaluate uncertainty; where rigorous evaluation is precluded by the method, a reasonable estimation is acceptable based on principles, experience, or validation data.

While the standard does not mandate a specific format or named "uncertainty budget," the practical implementation — widely expected by accreditation bodies — is an uncertainty budget: a detailed breakdown of uncertainty components, their quantification, and combination into a final expanded uncertainty.

This aligns with the Guide to the Expression of Uncertainty in Measurement (GUM – JCGM 100:2008), the foundational reference for uncertainty evaluation.

What Is an Uncertainty Budget?

An uncertainty budget is a tabular or documented summary that:

1. Lists all relevant sources of uncertainty affecting the measurement result.
2. Quantifies each source as a standard uncertainty (u).
3. Classifies them as Type A (evaluated by statistical methods, e.g., repeated measurements) or Type B (evaluated by other means, e.g., calibration certificates, experience, manufacturer specs).
4. Applies sensitivity coefficients (if needed) to convert to the measurand units.
5. Combines them — usually via the root-sum-of-squares (RSS) method — to obtain the combined standard uncertainty (u_c).
6. Multiplies u_c by a coverage factor (k) to yield the expanded uncertainty (U).

Why Uncertainty Budgets Matter

Uncertainty budgets deliver several key benefits:

• Ensure metrological traceability (Clause 6.5) by linking results to references through documented uncertainty chains.
• Support conformity decisions (Clause 7.8) — especially when assessing compliance with specifications or regulatory limits.
• Enable laboratories to identify and reduce dominant uncertainty sources, improving overall measurement quality.
• Meet accreditation body expectations (e.g., ILAC P14, PJLA PL-3) for documented uncertainty evaluation.

The uncertainty budget is far more than paperwork — it is the laboratory's transparent demonstration of measurement reliability. By systematically identifying, quantifying, and combining uncertainty components per ISO/IEC 17025:2017 and the GUM, laboratories build trust in their results and maintain technical competence.

For laboratories pursuing or maintaining ISO/IEC 17025 accreditation, mastering uncertainty budgets is essential. If your organization needs assistance with uncertainty evaluation, documentation, or training, contact MSG for expert guidance.