12. Comparing measurement results using measurement uncertainty estimates

Brief summary: This section explains that measurement uncertainty estimates are indispensable if we want to compare two measurement results, both having uncertainty.

Comparing measurement results using measurement uncertainty estimates
http://www.uttv.ee/naita?id=18095

https://www.youtube.com/watch?v=l6nYn6Pe7f0

Additional information. The above presented approach works well when comparing two results, both having uncertainty, with the aim of determining agreement between them. The approach can be to some extent used also in the case when one of the results has uncertainty estimate available but the second one does not. In that case the uncertainty of that second result can be taken as zero. This makes the test stricter. I.e., if the obtained zeta score still shows agreement then the results are in agreement, whatever the actual uncertainty of the second result. If the test shows disagreement or doubtful situation, then it is impossible to say whether there is agreement or not.

However, the presented approach is not suitable for another very important kind of comparison: deciding, whether a measured value (having uncertainty) is above or below a legal limit. In other words: compliance assessment. Probably the best approach to assess compliance of a result having uncertainty with permissible limits is the one presented in this Eurachem guide: Use of uncertainty information in compliance assessment. (2nd ed. 2021).

Very briefly, for deciding if an item can be accepted (is compliant) or rejected (is not compliant) it is necessary to:

1. Have analysis result with measurement uncertainty;
2. Have a specification giving the upper and/or lower permitted limits;
3. Define decision rules that describe how the uncertainty will be taken into account in compliance assessment. Such rules typically define acceptance and rejection zone limits and the respective probabilities of the actual values of measurands being within or outside the limits.

In some fields, for helping compliance assessment, the so-called target measurement uncertainty is defined. This is the maximum permissible uncertainty for the specific type of measurement or analysis.