Prof. Tariq A. Aldowaisan

Gulf Lead Consultants

Measurement Uncertainty in ISO 17025

TARIQ ALDOWAISAN Founder & Senior Consultant, Gulf Lead Consultants

Prof. Tariq A. Aldowaisan

• Ph.D. in industrial & management systems engineering (IMSE) from ASU, USA

• Professor of IMSE at the College of Engineering & Petroleum, KU

• Founder and senior consultant at Gulf Lead Consultants.

• Certified Quality Engineering (CQE), Certified Manager of Quality/ Organizational Excellence (CMQ/OE), and Certified Quality Auditor (CQA) from American Society for Quality (ASQ).

• Certified Safety Professional (CSP), Associate Safety Professional (ASP), and Certified Environmental, Health and Safety Trainer (CET) from Board of Certified Safety Professionals (BCSP).

• Has over 25 years experience in quality, organizational excellence, and safety.

What is a MU Estimate?

“parameter, associated with the result of a

measurement, that characterizes the dispersion of

the values that could reasonably be attributed to the

measurand”

This parameter is typically _______ or _________

SD Width of a CI

International Vocabulary of basic and general standard terms in Metrology (1993)

1

2

n

xXSD

Measurement result

Reference value

Precision (mu)

Bias

−𝑘𝑠 +𝑘𝑠

Result of Test or Measurement Method

Accuracy/ Error

Trueness Bias

Precision

Repeatability Reproducibility

Ref. ISO 21748:2010

Trueness/Bias (Systematic Error)… Difference between the expectation of a test result

or measurement result and a true value

Precision (Random Error)… Closeness of agreement between independent

test/measurement results obtained under stipulated conditions

Repeatability … Precision under repeatability conditions (same test procedure, same

operator, same test equipment under same conditions, same location, repetition over a

short period of time)

Reproducibility… Precision under reproducible conditions (same test method on

identical test items in different test facilities, with different operators using different

equipment)

Uncertainty

Ref. ISO 21748:2010

Uncertainty … is SD or a multiple of SD

Standard uncertainty … is SD of the result of a measurement

Uncertainty budget … is list of sources of uncertainties and their

associated standard uncertainties

Combined standard uncertainty … positive square root of sum of

variances or covariances of result’s affecting quantities

Expanded uncertainty … interval capturing a large fraction of the

values that could reasonably be attributed to the measurand

Coverage factor (k) … numerical factor used as a multiplier of the

combined standard uncertainty

MU Estimation Methods Two Approaches

MU Estimation Approaches

GUM Approach

Collaborative Study Approach

• The GUM is ________, yet __________.

• The Collaborative Study is less ___________, yet _________.

Therefore, in practice it is ____________ over GUM.

ideal difficult

rigorous easier

preferred

ISO Guide to the Expression of

Uncertainty in Measurement

(GUM)

‘bottom-up’ approach ‘top-down’ approach

The GUM Approach ‘predicts the uncertainty in the form of a variance on the basis of

variances associated with inputs to a mathematical model. The other (Collaborative

Study Approach) uses the fact that, if those same influences vary representatively

during the course of a reproducibility study, the observed variance is a direct estimate of

the same uncertainty.’ ISO 21748:2010, p.24.

ISO 17025 4.1 Organization

4.2 Management

System

4.3 Document Control

4.4 Review of request

and contacts

4.5 Subcontracting of Tests & Calibration

4.7 Service to the

Customer

4.8 Complaints

4.9 Control of Nonconforming Testing &Calibration Work

4.13 Control of records

4.12 Preventive Action

4.11 Corrective Action

4.10 Improvement

4.14 Internal Audits

4.6 Purchasing Services

and Supplies

5.2 Personnel

5.3 Accommodation and Environmental Conditions

5.4 Test and Calibration Methods and Method

Validation

5.5 Equipment

5.6 Measurement Traceability

5.7 Sampling

5.8 Handling Test and Calibration Items

5.9 Assuring the Quality of Test and Calibration

Results

5.10 Reporting of Results

5.1 General

MU in ISO 17025

5.4.6.3 Take all uncertainty components into account

5.4.5.3 MU is part of methods validation

33 Times

5.4.6.2 When not possible to rigorously estimate MU, …

5.4.6 Estimation of uncertainty of measurement

5.4.4 Estimation of MU in new tests

5.10.3.1 MU statement in test reports

Conclusion

1. Reporting a test result without MU is meaningless

2. MU is key to complying with ISO 17025