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Thread: Uncertainty of Measurement and Precision in Chemistry test

  1. #1 Uncertainty of Measurement and Precision in Chemistry test 
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    Hello all,
    I know some of this but obviously not all which is why I'm looking for help (= simple enough explanation for an idiot like me)

    In a lot of the tests in the laboratory there is a target set of 0.10ug/L and the allowed uncertainty of measurement for that target is set at <30% (0.03ug/L)

    I worked out how to calculate the uncertainty figure using the random and systematic errors and it turned out to be 12.8% so that was that, well within the spec. I thought. However this was identified as being outside the target because as well as the UoM the test precision was being recorded on our QC chart at 15.5% and I'm trying to understand how that can be. I know that the expected/accepted precision target should be 10% so would that be the specific reason for querying the UoM (?) or is it a combination of -

    0.10 (target) X 12.8% (UoM) X 15.5% (Precision) = 0.13187 ug/L i.e. applying the worse case of UoM and Precision to the target of 0.10ug/L the result would be 0.032ug/L which is further away than the required 0.030ug/L an so is greater than the 30% required (0.03ug/L) or am I off track?

    I'd really appreciate anyone that can give me an idiots explanation as I have to apply this to a number of other tests and I had been expecting to calculate the UoM, get a % result and move on. Instead I'm possibly looking at calculating a result each time (as I've tried above) using the UoM and Precision results instead of just identifying that % UoM calculation is within specification.




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