Measurement Uncertainty and Traceability: A Plain-English Guide for ISO 17025

Measurement uncertainty is the part of ISO/IEC 17025 that laboratories find hardest to explain and assessors find easiest to probe. Together with metrological traceability, it is what turns a number on a report into a result someone can rely on. This article explains both in plain terms, along with the decision rules that connect them to pass or fail statements.

Short answer

Measurement uncertainty quantifies the doubt around a result, so it can be reported as a value plus or minus a range at a stated confidence level. Metrological traceability links the result, through an unbroken chain of calibrations, to the SI units. ISO/IEC 17025 requires laboratories to evaluate uncertainty (clause 7.6), establish traceability (clause 6.5), and agree a decision rule when they state whether a result conforms to a specification (clauses 7.1.3 and 7.8.6).

Measurement uncertainty in practice

Every measurement has sources of doubt: the reference standard, the instrument’s resolution and drift, the method, environmental conditions, the sample, and the person doing the work. An uncertainty budget lists those sources, estimates each one, and combines them. The internationally accepted method is set out in the Guide to the Expression of Uncertainty in Measurement (GUM).

1. Define what is being measured and the equation that produces the result.
2. List the sources of uncertainty. Calibration, resolution, repeatability, reproducibility, environment, reference materials.
3. Quantify each one. Type A evaluations use statistics from repeated measurements; Type B evaluations use other information, such as calibration certificates and manufacturer specifications.
4. Combine them into a combined standard uncertainty.
5. Expand it by a coverage factor, usually k = 2 for approximately 95 per cent confidence.
6. Report it with the result, in the same units, where required.

Testing laboratories that cannot rigorously model every source often use validation and quality control data, such as reproducibility from intermediate precision studies and bias from reference materials or proficiency testing. That approach is accepted, provided it reflects the real performance of the method and is kept up to date.

What ISO 17025 requires

Metrological traceability

Traceability answers the question “how do you know your instrument is right?” Clause 6.5 requires results to be traceable to the SI units through a documented, unbroken chain of calibrations, each contributing to the uncertainty. In practice that means:

• Calibrations performed by competent laboratories, usually NATA accredited for the relevant measurement.
• Certified reference materials from competent producers, with stated values and uncertainties.
• Where traceability to SI units is not technically possible, traceability to an appropriate reference, such as a certified reference material or an agreed reference method.
• Reviewing each calibration certificate on receipt to confirm the results and uncertainty meet the laboratory’s requirements, not just filing it.

In Australia, the top of the chain for physical measurement is the National Measurement Institute.

Decision rules

When a customer asks whether a sample passes or fails a specification, uncertainty matters. A result just inside the limit with a large uncertainty might actually be outside it. A decision rule sets out how that is handled, for example whether a guard band is applied, and who carries the risk of a wrong decision. ISO/IEC 17025 requires the rule to be agreed at contract review and stated in the report, unless the rule is already built into the specification or standard.

Where laboratories get caught out

• Uncertainty budgets that cannot be explained by the people who use them.
• Budgets that were never updated after a change of equipment, method or reference material.
• Calibration certificates accepted without checking the uncertainty is fit for purpose.
• Pass or fail statements with no agreed decision rule.
• Testing laboratories that assume uncertainty is only a calibration laboratory requirement.

The full set of requirements is in our ISO 17025 requirements checklist, and the assessment process is covered in the NATA accreditation guide.

How Streamline helps

We help laboratories set up practical uncertainty budgets, traceability records and decision rules that their own people can explain to an assessor. Accreditation itself is granted by NATA; our role is getting the laboratory ready for assessment and keeping the system working afterwards. Read about our ISO 17025 consulting, or talk to us about your laboratory.

More ISO 17025 guides

• ISO 17025 consulting for laboratories
• NATA accreditation: process, timeframes and costs
• ISO 17025 Option A vs Option B
• ISO 17025 requirements, clause by clause

Frequently asked questions

What is measurement uncertainty?

Measurement uncertainty is a quantified range around a result within which the true value is believed to lie, at a stated level of confidence. It reflects all the known sources of doubt in a measurement, such as equipment, reference standards, method, environment and operator.

Does ISO 17025 require testing laboratories to report measurement uncertainty?

Testing laboratories must evaluate measurement uncertainty. They report it when it is relevant to the validity or application of the result, when the customer asks for it, or when it affects conformity to a specification limit. Calibration laboratories report uncertainty with every calibration result.

What is a coverage factor of k = 2?

The combined standard uncertainty is multiplied by a coverage factor to give the expanded uncertainty. A coverage factor of k = 2 gives a level of confidence of approximately 95 per cent for a normal distribution, which is why it is the most common choice in reports.

What is metrological traceability?

An unbroken chain of calibrations linking a measurement result to a reference, usually the SI units, with the uncertainty known at each step. In Australia, the chain usually runs through NATA accredited calibration laboratories to the National Measurement Institute.

What is a decision rule?

A rule that describes how measurement uncertainty is taken into account when stating whether a result conforms to a specification. ISO/IEC 17025 requires the decision rule to be agreed with the customer when a statement of conformity is requested, unless it is inherent in the specification or standard.