Variance propagation with uncertainty and variability separated

8 Variance propagation with uncertainty and variability separated 

So far in the case-study, we have focused on variance propagation methods and have not made an effort to distinguish between the relative contributions to overall variance from uncertainty and variability. In the examples above, the cumulative distributions presented in figures all reflect overall variance that includes the combined contributions from both uncertainty and variability. So our last step is to illustrate a two-dimensional analysis in which we distinguish and display separate contributions from uncertainty and variability. We begin this analysis by going back to our inputs and assessing the relative contributions from uncertainty and variability. 

First we consider the concentration, Cw, of PBLx in surface water. Previously in this annex, we showed that the observed ocean and freshwater data for concentration could be fit to a 

Next we consider the bioconcentration factor, BCF, of PBLx. Previously in this annex, we used results from a series of experiments to develop for BCF a probability distribution that includes both variability and uncertainty. Again, an assumed evaluation of the data and measurements indicates that only 40% of the observed variance is due to variability and the remaining 60% of the observed variance is attributable to uncertainty. So the concentration data consist of a family of distributions with a variance having a geometric standard deviation of 1.93 due to variability. These curves have a location range that spans a range with a geometric standard deviation of 2.37. 

Finally, we consider the data on fish ingestion. Here we note that essentially all of the variance in the probability distribution used to represent observations on fish consumption reflects variability. So in this case, the curve reflecting both variability and uncertainty is assumed to be the same as the curve reflecting variability. 

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A2.3.8 Variance propagation with uncertainty and variability separated... 

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