In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Measurement Uncertainty

Alex Williams

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Uncertainty

Is a fundamental property of a result It is not an optional extra It is not just as an additional burden All results have an uncertainty on their

value Needs to be evaluated irrespective of

requirements of 17025

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Overview

What is meant by Uncertainty? What information is it intended to give ? Why is it important? How is uncertainty evaluated?

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

What is Uncertainty?

There will always be an uncertainty about the value of a result

Even when correction factors have been applied

Because there will be an uncertainty on these factors

There will also be an uncertainty arising from random effects.

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Lab A Lab B Lab C Lab D Lab E

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Lab A Lab B Lab C Lab D Lab E

Uni

ts

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Why Uncertainty is Important

to assess the reliability of the result to know the confidence that can be

placed in any decisions based on its use in order to compare measurement

results

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Uncertainty should be Quantified in a

Way that is Universal:

applicable to all kinds of measurements Internally consistent:

independent of how components are grouped Transferable:

use uncertainty on a result in derivation of uncertainty on dependant results

Procedures set out in: Guide to the Expression of Uncertainty in Measurement (GUM)Published by ISO ISBN 92-67-10188-9 1993

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Uncertainty of Measurement - Definition

parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Uncertainty of Measurement

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Standard Uncertainty Uncertainty of the result expressed as a

Standard Deviation

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Expanded Uncertainty ..interval about the result of a measurement

that may be expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand.

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Principles of Measurement

Measurement results are obtained from a comparison of the unknown with a standard

In the simplest case, the value VU of the unknown is VU=f(V1, IU, I1),Where I1 indication, when measuring the standard,

IU when measuring the unknown and

V1 the value of the standard

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Evaluating Uncertainty

Uncertainty on result is:- Uncertainty from comparison with standard. Plus the uncertainty on the value of the

standard. Many sources of uncertainty associated with

the comparison Uncertainty on standard usually small Traceability is to the value of the standard

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Measurement of Massusing two pan balance

Effects to be taken into account: buoyancy equality of arm lengths sensitivity to position on pan air temperature gradients electrostatic forces

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Evaluating UncertaintySources of uncertainty

1. Incomplete definition of the measurand.

2. Sampling - the sample measured may not be representative.

3. Incomplete implementation of the measurement method.

4. Personal bias in reading analogue instruments.

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Evaluating UncertaintySources of uncertainty

5. Inadequate knowledge of the effects of environmental conditions on the measurement procedure or imperfect measurement of environmental conditions

6. Instrument calibration uncertainty

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Evaluating UncertaintySources of uncertainty

7. Instrument resolution or discrimination threshold.

8. Values assigned to measurement standards and reference materials.

9. Values of constants and other parameters obtained from external sources and used in the data reduction algorithm.

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Evaluating UncertaintySources of uncertainty

10. Instrument approximations and assumptions incorporated in the measurement method and procedure.

11.Variations in repeated observations of the measurand under apparently identical conditions.

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Evaluating Uncertainty

Approach depends on information available

Cause and effect diagram helps track components.

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Preparation of a Calibration Standard

V

PmcCd

1000

PurityV

m

Repeatability

Calibration

Temperature

c(Cd)

m(tare) m(gross)

Repeatability Repeatability

Calibration

Linearity

Sensitivity

Calibration

Linearity

Sensitivity

ReadabilityReadability

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Pesticide in Bread

P(op)

I(op) c(ref) V(op)

m(sample)I(ref)

V(op)

Calibration

Temperature

dilution

dilutionCalibration

V(ref)

V(ref)CalibrationTemperature

m(ref)

Calibration

m(ref)

Purity (ref)I(op)

Calibration

Recovery

m(gross)

I(ref)

CalibrationCalibration

Linearity m(tare)

m(sample)

Calibration

Linearity

F(hom)

Repeatability

Linearity

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Evaluating Uncertainty

For many types of test, method validation provides most of required information

Identifies and controls parameters that affect result

Plus normal QA

In: Wenclawiak, Koch, Hadjicostas (eds.) Quality Assurance in Analytical Chemistry – Training and Teaching

Williams, A.: Measurement Uncertainty © Springer-Verlag Berlin Heidelberg 2003

Conclusions

Uncertainty is an essential component of the result

Necessary to ensure comparability of results

In many cases method validation studies & QA data provide most of information required

More information onwww.measurementuncertainty.org