Frequently asked questions

Non-negative parameter characterizing the dispersion of the quantity values being attributed to a measurand, based on the information used (International vocabulary of metrology — Basic and general concepts and associated terms, VIM).

Yes, measurement results cannot be perfect. All analytical results vary because the measurement conditions fluctuate. A large part of the measurement uncertainty is also due to the uncertainty as to whether the sample was really taken in a representative and unbiased manner.

No, on the contrary: The knowledge of the uncertainty increases the confidence we can have in measured values. Only when we know how uncertain (or better: how certain) the measured values are, we also know how strongly we can load such measured values.

This does not have to be the case! The quality of an analytical result is good and sufficient if the requirements for the certainty of the value in the specific case are met. Measurement with the smallest possible uncertainty in all cases would make analysis unnecessarily expensive.

If there are no legal requirements, this requires an assessment of what consequences a deviating analysis value would have. The customer must decide what is still tolerable for him. Of course, the analytical possibilities must be taken into account. It must also be borne in mind that increasing accuracy (and thus decreasing measurement uncertainty) is almost always associated with increasing effort and thus increasing costs. A discussion between the customer and the analyst is often necessary to clarify this issue.

Many important decisions are based on the results of chemical quantitative analysis; the results are used, for example, to estimate yields, to check materials against specifications or statutory limits, or to estimate monetary value. Whenever decisions are based on analytical results, it is important to have some indication of the quality of the results, that is, the extent to which they can be relied on for the purpose in hand. (Eurachem/CITAC - Guide Quantifying uncertainty in analytical measurement)

You can find out more here.

There are a large number of documents on this. The most important ones in our opinion are:

• ISO Guide to the expression of uncertainty in measurement - the GUM is the "bible" of measurement uncertainty estimation
• EURACHEM/CITAC-Guide “Quantifiying Uncertainty in Analytical Measurement”  - is an interpretation of the GUM for chemical  analysis
• ISO 11352 - Water quality - Estimation of measurement uncertainty based on validation and quality control data - a pragmatic approach using results from control charts, reference material analysis etc. An associated EXCEL evaluation sheet can be found on our download page
• NORDTEST: Handbook for calculation of measurement uncertainty in environmental laboratories. Report TR 537 (www.nordtest.org)

You can find more documents here.

For this purpose, EURACHEM, CITAC, EUROLAB, NORDTEST and the UK RSC Analytical Methods Committee have developed a guidance document: Measurement uncertainty arising from sampling - A guide to methods and approaches.

An associated EXCEL evaluation sheet can be found on our download page

Each specification of a measurement uncertainty also includes the specification of the confidence level used. 95% confidence level means that the "true" value of the measurand is expected with 95% probability in the specified range. If a standard deviation is specified (this is then referred to as a standard uncertainty), then the confidence level is only approx. 68%. By multiplying the standard uncertainty by a coverage factor k, one obtains an expanded uncertainty with a higher confidence level. The most commonly used coverage factor k=2 corresponds approximately to a confidence level of 95%. The specification of a measurement uncertainty without a confidence level is worthless!