Uncertain about uncertainty?

Making an estimate of measurement uncertainty is not just about satisfying some clause in some standard and making it through your next technical audit, it is about gaining a better understanding of your measurement.  Note: the requirement that testing and calibration laboratories estimate the uncertainty of their measurements is covered under clause 5.4.6 of ISO/IEC 17025 General requirements for the competence of testing and calibration laboratories. 

The current definition of uncertainty taken from the international vocabulary of metrology (VIM) defines measurement uncertainty as:

'Non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand based on the information used'

This requirement tells laboratories that they need to take the time to look critically at the information they have used to estimate uncertainty of measurement. Clause defines where this information is going to come from:

’…In these areas the laboratory shall at least attempt to identify all the components of uncertainty and make a reasonable estimation, and shall ensure that the form of reporting of the result does not give a wrong impression of the uncertainty.  Reasonable estimation shall be based on knowledge of the performance of the method and on measurement scope and shall make use of, for example, previous experience and validation data.’

If you are using a validated method and have your validation data, then you have all the information you need to estimate the uncertainty of your method. Fortunately there are a number of guidance documents written specifically to assist chemical laboratories to use the information they have already collected to estimate their uncertainty. These include the Eurachem Quantifying Uncertainty in Analytical Measurement guide and the Hand book for the Estimation of Measurement Uncertainty in Environmental Laboratories produced by Nordtest.

Once an estimate of measurement uncertainty has been made and before any results are reported, laboratories need to make sure their estimate makes sense. Check if the expanded uncertainty matches with the variability that is seen when analysing duplicates, or match the variability seen for control samples analysed over time. It is worth noting that estimating measurement uncertainty is not overly complex and by following a few simple rules you will obtain an estimate that meets all the requirements.

If you still need assistance after reading all the documents on measurement uncertainty, you may wish to attend one of NMI’s training courses on measurement uncertainty. We have specialist courses for physical, chemical and biological measurements. If you would like more information please visit our website or contact us by email at This email address is being protected from spambots. You need JavaScript enabled to view it.

Upcoming NMI training courses for 2015

Statistics for Metrology


26 February

Physical Measurement Courses


Introduction to Estimating Measurement Uncertainty


3 March
26 March

Time and Frequency Measurement


4-5 March

Fundamentals of Geometrical Dimensioning and Tolerancing


12-14 May

Humidity Measurement


24 February

Chemical and Biological Measurement Courses


Analytical Method Validation


10-11 March

Estimating Measurement Uncertainty for Biologists


23 April

Estimating Measurement Uncertainty for Chemists


12–13 March