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Measurement uncertainty - frequently asked questions

What is measurement uncertainty?

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, 3. Auflage)

Uncertainty - is it a must?

Yes, measurement results cannot be perfect. All analytical results vary, because measurement conditions are fluctuating. A major part of the uncertainty also is due to the uncertainty of the sampling, whether the sample is really representative and not adulterated in the sampling process.

Does reporting of measurement uncertainties mean that there are doubts on the validity of the results?

No, on the contrary: Knowledge of uncertainty increases the confidence we can have in the results. Only if we know how uncertain (or better: how certain) the results are, we can know how reliable the data are in the interpretation.

Is the laboratory with the lowest uncertainty the best laboratory?

Not necessarily! The quality of the analytical result is ok and sufficient, if the quality requirements are met that have to be applied in this special case. Measurements with the lowest achieveable uncertainty in all cases would unnecessarily incereas the costs of the analysis.

Where to find the suitbale quality requirements for the analysis?

If there is no legal requirement, we need an estimate on the consequences of a deviating analytical results. The customer has to decide what risk is acceptable to him. Of course the analytical possibilities have to be taken into account. It also has to be considered that increasing accuracy (i.e. decreasing measurement uncertainty) usually is accompanioed with increasing effort and therefore also increasing costs. In many cases a dialogue between customers and analyst is necessary to clarify these questions.

Why is measurement uncertainty important?

Man important decisions are based on the results of quantitative chemical analyses; e.g. these results are used to calculate yields, to test materials against specifications or legal limits or to specify the monetary value. Whenever decisions are based on analytical results, information on the quality, i.e. the reliability of these is indispensable and should be quantified.

How can I use the measurement uncertainty in a compliance assessment?

Please find more oin this topic here

Which documents are describing the estimation of measurement uncertainty?

There are alot of documents. In our opinion the following ones are the most relevant:

Please find more documents here.

How can I quantify the unceratinty in sampling?

EURACHEM, CITAC, EUROLAB, NORDTEST and the UK RSC Analytical Methods Committee developed a guide on this topic: Measurement uncertainty arising from sampling - A guide to methods and approaches

What means confidence level?

The confidence level is an indispensable information for all measurement uncertainty reporting. 95% confidence range means, that the "true" value of the measurand is expected with 95% probability in the reported range. If a standard deviation is reported (called standard uncertainty) the confidence level is only approx. 69%. By multiplying the standard uncertainty with a coverage factor k an expanded uncertainty with a higher level of confidence is obtained. The commonly used coverage factor k=2 corresponds to a confidence level of about 95%. Reporting measurement uncertainties without stating the confidence level is absolutely worthless!

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