Understanding Measurement Uncertainty According to NABL ISO/IEC 17025

In testing and calibration laboratories, the accuracy of results is not absolute; it always carries a degree of uncertainty. This doubt is known as Measurement Uncertainty (MU).

According to ISO/IEC 17025, laboratories accredited by the NABL (National Accreditation Board for Testing and Calibration Laboratories) in India are required to estimate, evaluate, and report measurement uncertainty for all accredited parameters.

Understanding MU is not only a compliance requirement but also a scientific necessity. It allows customers, regulators, and auditors to assess the reliability of reported results.

What is Measurement Uncertainty?

Measurement Uncertainty is a quantitative expression of doubt in a measurement result.

According to ISO/IEC 17025:

“Measurement uncertainty is a parameter associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand.”

Simply put, every measurement has errors due to factors such as instrument, environment, operator, or method. MU quantifies this combined doubt in a standard way.

Why is Measurement Uncertainty Important in NABL ISO/IEC 17025?

NABL requires laboratories to evaluate MU for several reasons:

• Regulatory Compliance: Mandatory requirement under ISO/IEC 17025 Clause 7.6.

• Customer Confidence: Provides clients with a reliable range of results.

• Comparison of Results: Enables meaningful comparison of measurements across labs.

• Risk Management: Helps decision-making in conformity assessment (pass/fail).

• Audit Preparedness: NABL auditors place a strong emphasis on MU estimation during assessments.

Without MU, a reported result is incomplete and may be misleading.

Key Concepts in Measurement Uncertainty

1. Measurand

   • The specific quantity subject to measurement (e.g., voltage, temperature, pH).

2. Sources of Uncertainty

   • Instrument accuracy and calibration.

   • Environmental conditions (temperature, humidity, vibration).

   • Operator skill and repeatability.

   • Sample variability.

   • Reference standards traceability.

3. Types of Uncertainty

   • Type A Evaluation: Statistical analysis of repeated measurements.

   • Type B Evaluation: Based on manufacturer specifications, calibration certificates, or published data.

Steps to Estimate Measurement Uncertainty

According to NABL and ISO/IEC 17025 guidelines, the general process includes:

1. Define the Measurand

Clearly specify what is being measured. Example: The temperature of a sample is 25°C.

2. Identify Sources of Uncertainty

List all possible contributors (instrument resolution, calibration certificate error, repeatability, environment).

3. Quantify Uncertainties

• Type A: Perform repeated measurements and calculate the standard deviation.

• Type B: Use instrument specifications, reference standards, or previous calibration data.

4. Combine Uncertainties

Use the root-sum-square (RSS) method to combine Type A and Type B components.

uc=u12+u22+u32+…u_c = \sqrt{u_1^2 + u_2^2 + u_3^2 + …}uc​=u12​+u22​+u32​+…​

where ucu_cuc​ = combined standard uncertainty.

5. Expand the Uncertainty

Multiply combined uncertainty by a coverage factor (k), usually k=2 for 95% confidence.

U=k×ucU = k \times u_cU=k×uc​

6. Report the Result with MU

Final result must be expressed with uncertainty. Example:

$$25.0°C\pm 0.3°C(k=2,95\%confidence)$$

Example of Measurement Uncertainty in Calibration

Suppose a laboratory calibrates a 100 g weight. Sources of uncertainty may include:

• Balance resolution: ±0.1 mg

• Calibration certificate of balance: ±0.2 mg

• Environmental effects: ±0.15 mg

• Repeatability: ±0.05 mg

After calculating combined uncertainty and applying k=2, the result may be reported as:

$$100.000g\pm 0.25mg(k=2,95\%)$$

This gives the customer confidence that the true value lies within this range.

NABL Requirements for Measurement Uncertainty

NABL guidelines (NABL 141, NABL 162, NABL 164) emphasize:

• Each accredited parameter must have an MU budget.

• MU must be calculated scientifically and documented.

• Records should include:

  • Identified sources of uncertainty.

  • Method of estimation (Type A/Type B).

  • Combined and expanded uncertainties.

  • Traceability to standards.

• MU must be reviewed periodically and updated if conditions change.

During audits, NABL assessors often raise non-conformities when:

• MU calculations are missing.

• Only Type A or Type B is considered (not both).

• No evidence of traceability for values used.

• Results reported without MU.

Challenges Labs Face in Measurement Uncertainty

• Complex statistical calculations.

• Missing calibration traceability records.

• Lack of trained staff in uncertainty estimation.

• Manual errors in spreadsheets.

• Difficulty in updating MU budgets for multiple parameters.

Role of Calibration Management Software in MU Compliance

Manually calculating and tracking MU for every parameter is prone to errors. Calibration Management Software automates and streamlines the process:

• Automated Uncertainty Budgets – Build and maintain MU templates.

• Integration with Calibration Records – Direct link between MU and equipment certificates.

• Audit-Ready Reports – Generate MU documentation instantly for NABL assessors.

• Error Reduction – Reduces mistakes compared to manual spreadsheets.

• Real-Time Updates – Easily revise MU values when equipment, methods, or conditions change.

Explore Zeptac Calibration Software to simplify measurement uncertainty management and ensure NABL ISO/IEC 17025 compliance.

FAQs on Measurement Uncertainty in NABL ISO/IEC 17025

1. What is measurement uncertainty in simple words?
It is the quantified doubt about a measurement result, showing the range within which the true value is expected to lie.

2. Is measurement uncertainty mandatory for NABL accreditation?
Yes, NABL requires all accredited parameters to have an evaluated and documented MU budget.

3. How do you calculate measurement uncertainty?
By identifying sources of error, evaluating Type A (statistical) and Type B (reference data) components, combining them, and expanding with a coverage factor (k).

4. What is the confidence level used in MU reporting?
Most laboratories use a 95% confidence level with k = 2, as per the ISO Guide to the Expression of Uncertainty in Measurement (GUM).

5. How can software help in MU estimation?
Calibration management software automates MU calculations, links them with calibration data, and generates audit-ready reports, reducing errors and saving time.

Conclusion

Measurement uncertainty is not just a mathematical concept—it is the backbone of ISO/IEC 17025 compliance and NABL accreditation. It provides transparency, reliability, and confidence in reported results.

For laboratories, the challenge lies in consistent, accurate, and audit-ready MU documentation. Instead of relying on error-prone manual methods, adopting Calibration Management Software ensures efficiency, accuracy, and compliance.