measurement uncertainty in simple
A chemical reaction is a probabilty of each molecules abilty to get into a reaction with the other. Hence all reaction is occured within the limits of this probability.
A common way of presenting the different contributions to the total measurement uncertainty is to use a socalled fishbone (or causeandeffect) diagram. We propose a model, where either the reproducibility withinlaboratory (Rw ) is combined with estimates of the method and laboratory bias, or the reproducibility sR is used more or less directly, ISO Guide 21748/8/. The alternative way is to construct a detailed fishbone diagram and calculate/estimate the individual uncertainty contributions. This approach may prove very useful when studying or quantifying individual uncertainty components.
It has been shown, though, that in some cases this methodology underestimates the measurement uncertainty /3/, partly because it is hard to include all possible uncertainty contributions in such an approach. By using existing and experimentally determined quality control (QC) and method validation data, the probability of including all uncertainty contributions will be maximised.
What is measurement uncertainty?
It is a number which is used after a result ±
All measurements are affected by a certain error. The measurement uncertainty tells us what size the measurement error might be. Therefore, the measurement uncertainty is an important part of the reported result
ISO Definition: Measurement uncertainty is ”A parameter associated with the result of a measurement, that characterises the dispersion of the values that could reasonably be attributed to the measurand”
Who needs measurement uncertainties?
The clinician needs it together with the result to make a correct diagnosis. The uncertainty of the result is important, e.g. when looking at the limits of reference interval or cut off
The laboratory wants to know its own quality of measurement and to improve to the required quality
Why should the laboratory give measurement uncertainty?
As explained above, the clinicians need it to make correct decisions.
An estimation of the measurement uncertainty is required in ISO 17025, ISO 15189 and in the other accreditation standarts. Recently in Canada all laboratories are obliged to calculate their uncertainty of measurement for each analyte.
How is measurement uncertainty obtained?
The basis for the evaluation is a measurement and statistical approach, where the different uncertainty sources are estimated and combined into a single value
“Basis for the estimation of measurement uncertainty is the existing knowledge (no special scientific research should be required from the laboratories). Existing experimental data should be used (quality control charts, validation, interlaboratory comparisons, CRM etc.)”
Guidelines are given in GUM /1/, further developed in, e.g., EA guidelines /12/, the Eurachem/Citac guide /2/, in a Eurolab technical report /3/ and in ISO/DTS 21748 /8/
How is the result expressed with measurement uncertainty?
Measurement uncertainty should normally be expressed as U, the combined expanded measurement uncertainty, using a coverage factor k = 2(1.96), providing a level of confidence of approximately 95 %
It is often useful to state how the measurement uncertainty was obtained
Example, where ± 7 is the measurement uncertainty:
How should measurement uncertainty be used?
It can be used as in, to decide whether there is a difference between results from different laboratories, or results from the same laboratory at different occasions (time trends etc.)
It is necessary when comparing results to allowable values, e.g. tolerance limits or allowable (legal) concentrations
How is the result expressed with measurement uncertainty?
