Uncertainty: A Standard Measurement Principle 

Fundamental measurement principles recognize that all measurements are comparisons to a reference. The quality of that comparison depends on: 

• The traceability of reference standards 
• The stability and resolution of the instrument under test 
• The calibration method and measurement model 
• Environmental and repeatability effects 

Measurement uncertainty integrates all of these contributors into a single, defensible statement. 

From a technical standpoint, uncertainty answers critical questions: 

• How reliable is this measurement? 
• Can this result support a conformity decision? 
• Is this instrument fit for its intended use? 

Without uncertainty, measurement results cannot be properly interpreted, compared, or used to make risk-based decisions.  

Why Measurement Uncertainty Matters to Quality Managers 

For quality managers, uncertainty is not an abstract metrology concept—it is a risk management tool. 

When evaluating a commercial calibration supplier, uncertainty directly impacts: 

1. Compliance and Audit Readiness

ISO/IEC 17025, ANSI/NCSL Z540, and most regulated quality systems require that calibration results include stated measurement uncertainty. Certificates that omit uncertainty—or provide unexplained, non-traceable, generic values—may not withstand audit scrutiny. 

2. Decision Rules (Pass/Fail) and Tolerance Decisions

Calibration results are often used to determine whether equipment meets specification. Measurement uncertainty affects guard banding, decision rules, and false accept/reject risk. A supplier with poor or inflated uncertainty may mask real nonconformities—or create unnecessary failures. 

3. Fitness for Use

An instrument can technically be “in tolerance” and still be unsuitable for its application if its measurement uncertainty is too large relative to the process requirements. Quality managers must ensure that calibration uncertainty supports actual measurement needs, not just nominal specifications. 

4. Supplier Competence

A credible calibration provider can clearly explain: 

• How uncertainty is calculated 
• Which contributors are included 
• How traceability is maintained 
• How uncertainty relates to the reported result 

If a supplier cannot defend their uncertainty statements, their technical competence should be questioned.  A benefit to utilizing an ISO 17025 accredited laboratory for external calibration is that technical competence is evaluated and audited by a third party and that laboratories measurement capabilities are provided publicly on their scope of accreditation. 

What to Look for in a Calibration Supplier 

When reviewing calibration certificates or selecting a provider, quality managers should look for: 

• Clearly stated expanded uncertainty with confidence level (typically k = 2, ~95%) 
• Uncertainty values appropriate to the measurement and equipment used 
• Measurement uncertainty that is not reported smaller than that labs scope of accreditation 
• Statement of traceability to national or international standards 
• Technical transparency—not boilerplate language 

Measurement uncertainty is not an administrative detail. It is central to measurement integrity. 

Closing Perspective 

Measurement uncertainty defines the credibility of a measurement result. For organizations that rely on data to make technical, regulatory, or commercial decisions, understanding uncertainty—and demanding it from calibration suppliers—is essential. 

At its core, good metrology is not about producing numbers. It is about understanding how much confidence those numbers deserve.