GENERAL INFORMATION ON MEASUREMENT UNCERTAINTY
For those relying on test results to make informed decisions and judgements.

Measurement Uncertainty
Many important business decisions are based on the results obtained from quantitative testing. It
is important that an indication of the quality of laboratory test results reported is available to you.
Results of tests cannot be perfect and the term measurement uncertainty is used to describe thislack of perfection. IANZ accredited laboratories are required to have a measure of the confidence
placed on each result to demonstrate its fitness for your purpose.
Measurement uncertainty is a vital part of the test result that gives this information, and one thatis used internationally.

The Testing Process
Inherent imperfections in each step of the testing process, from sampling to the final
measurement, cause variability in the results of the tests.
IANZ accredited laboratories make measurements and perform controls regularly to ensure that
this overall variability is small enough for the end result to be appropriate for your requirements.

Results should be fit for the intended purpose
If the uncertainty of a test result is too large, you will not be able to make a reliable decision. If it
is smaller than you require, the test may be unnecessarily complex and costly. Test result
uncertainties should be fit for the intended purpose.
If you are unsure as to what level of uncertainty you need, do not hesitate to contact thelaboratory.

What it could look likeIn the test result you receive, you will be given the normal information about what has beenmeasured and the units of measurement.
When a test result is presented as a measured value and a measurement uncertainty, it
prescribes an interval within which the true value of the quantity being measured is expected tolie with a stated level of confidence (usually 95%). This uncertainty interval varies in size,depending on the test.

General Information on Measurement Uncertainty• Measuredvalue

Large
Uncertainty

Uncertainty Interval

Uncertainty Interval

Uncertainty Interval

•

Small
Uncertainty

ExampleThe laboratory has found the lead content of a sample is 1.65 ± 0.15 mg kg -1. This means that the true valueof the quantity being measured is expected to be between 1.50 and 1.80 with (usually) 95% confidence.
Example Report
Total lead content (Pb):
1.65 mg kg -1
Measurement Uncertainty: ± 0.15 mg kg -1
The stated uncertainty is an expandedmeasurement uncertainty for a 95% level of confidence (standard deviation x2).

It will be easier to compare results
Most laboratories have until now chosen not to state measurement uncertainty in their test
reports. Instead, such information has been given only when the customer has asked for it.
In the future, information about the measurement uncertainty may appear more frequently inIANZ accredited test reports. It is also possible that you will come across new and unfamiliar
quality terms. This is because there are new international guides and standards describing
uncertainty.

SamplingYou may be wishing to have information on specific characteristics of a larger bulk of material
from which a test sample was taken.
The material may not be homogeneous.
The uncertainty of the reported test result will usually not include variations associated with lackof homogeneity of the bulk material. Often variations across the bulk of material will be very muchgreater than the test result uncertainty.
If you require information about bulk material variations, the laboratory will need to design thesampling plan to make the necessary estimates.

2General Information on Measurement Uncertainty

Uncertainty and Product Specification Values
Many tests are done to assure you that a characteristic of a product does not exceed a regulatoryor customer specification value. Without information about the measurement uncertainty, it may
appear to be easy to make compliance decisions but these decisions may be unreliable.
There will be costs involved when rejecting product that should have been accepted. There may
be judicial, medical, safety or trade consequences of accepting product or results that shouldhave been rejected.
Ask the laboratory how confident they are that the test item passed or failed the productspecification.
Example of applying the confidence interval to test results

A•

BValue ScaleC•

D•
•

E

•

hProduct value
specified(highestacceptablevalue)

Test result A indicates there is more than 95% confidence that the product failed the specification.
Test result E at 95% confidence indicates an acceptable product.
For test results B, C and D, it cannot be said with 95% confidence whether the product passed or failed.

More Information
The laboratory has experts ready to advise on all matters regarding sampling, test methods anduncertainty. Please talk with them about your particular tests and how to correctly interpret theresults.

Acknowledgement:This information guide is based on information provided
by
SP Swedish National Testing and Research InstituteProduced by:International Accreditation New Zealand
Private Bag 28908, Remuera, Auckland 1541
Tel: 09 525 6655 Fax: 09 525 2266
Email: info@ianz.govt.nz. www.ianz.govt.nz
AS IG3, October 2003
Reformatted and reprinted October 20073