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Technical issues related to the ERICA Tool (on Facilia website)

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Posted: 03/12/2010

What distribution

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should I use?

From:
Marko Černe
Jožef Stefan Institute
Slovenia

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Answer:
Easy question first your mean estimate when using a mean and pdf in Tier 3 of the ERICA Tool will be similar to estimate using Tier 2 and inutting inputting a mean value only; it will not be exactly the same as Tier 3 is probabilistic. The mean estimates using either a lognormal or exponential distribution in Tier 3 will be similar. The percentiles, minimum and maximum values will be different and dependent on the SD used for the lognormal distribution_

Which should you use? As is often the case there is no definitive answer to this. It depends what your criteria are - do you want to ensure your assessment is conservative? If yes then try both distributions in the Tool and compare your answers - it is relatively straightforward to do.

You appear to be questioning how representative your input data are? I'm afraid that you are best laced to answer this yourself. However, if your data are from close to your release source then you can have some confidence that they will give a conservative assessment downstream. If your data are distant from your source and you want to ensure populations upstream from your sampling site are protected then you need to obtain more samples form approriate locations or make predictions using a dispersion model.

Answered by Nick Beresford 08/12/2010

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