Brief summary: The first principle of measurement uncertainty is: the measurand must be correctly and unambiguously defined. The importance of measurand definition is explained on the example of pesticide determination in oranges.
Defining the measurand
http://www.uttv.ee/naita?id=17585
https://www.youtube.com/watch?v=Bf_0EssHQ60
Defining the measurand in the case of pesticide determination in oranges is not trivial. On one hand it is important to define whether the result is applied to a single orange or few oranges that were taken as the sample or whether it is applied to the whole lot of oranges (the whole analysis object, also called sampling target). On the other hand, oranges are not homogenous. Pesticides are applied on orange surface, not inside. At the same time pesticide can diffuse from the orange peel to the inside. So, a number of different possibilities exist: whole orange, whole peel, outside part of the peel, only orange flesh.
Combining together we get 8 possibilities, in what exactly we can determine pesticides. Measuring pesticide content according to any of these will lead to different and mutually non-comparable results.
In addition (not explained in the video) instead of defining the measurand via the total analyte content in the sample (or part of the sample) it is often more practical to look at some part of the analyte only.
A good example is phosphorus determination in soil. Although it is possible to determine the total phosphorus content in soil it is in fact more interesting to determine only the part that is available to plants – the bioavailable phosphorus – because it is this part of the total phosphorus content that contributes to the fertility of the soil and is therefore of interest in agriculture.
Total phosphorus content and bioavailable phosphorus content are different measurands and their values for the same soil differ strongly. This has important implications for the measurement procedure. This is explained in section 5.2.
