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Probe for EPMA / Re: Correction of negative result in quant
« Last post by Probeman on Today at 09:26:17 AM »What John says is correct. I'd like to say one more thing. It is totally appropriate as a final step to average together a series of acquisitions for a single element and then set the average value to zero if it is less than zero for reporting purposes. Be sure to continue propagating
uncertainties so you have a sense how negative the value is to check if your backgrounds etc. are reasonable.
So you are saying that if one zeros a negative average, one should still report the variance which includes the negative individual points? Hadn't thought about it, but that makes sense. Then everyone could see that the range of the variance drops below zero. Though I think I'd still leave the negative value as it is and let the user decide to do that (or not) in their reporting.
Also: It is also necessary to set negative values to zero to compute quantities like the analytical total or matrix correction factors. It can be subtle when to or not to set values which can not be less than zero to zero. Usually the answer has to do whether you will subsequently average the values together - in which case, average first then truncate.
Yes, I agree. For matrix corrections we set negative k-ratios to a very small number (10^-9 I think) because not only can we not take the SQRT of a negative number, sometimes having an actual zero causes problems in other calculations.
But I'm still thinking about the necessity of not including negative concentrations in the analytical total. I have two problems with this: first the user may just look at the analytical total and think that it is OK and overlook a specific element or two that is quite negative. And second, it bothers me that a column or row of concentrations would no longer (literally) add up to the printed total.
I think seeing negative k-ratios or concentrations (or low analytical totals) should (hopefully!) prompt the user to look more closely into problems in their analytical setup.