Hi Scott, if I understand correctly, the absolute uncertainties in the sizes are still large when you fit sizA and delta_size=(sizeB-sizeA), but I think that the uncertainty in the parameter delta_size showing the difference between the two could be very small, showing that there is a correlation between sizeA and sizeB. In order get this behavior I think you have to leave sizeA unconstrained when you do the multiple-dataset fit. If the uncertainty in delta_size is small, then the absolute uncertainties will be approximately sizeA +- ds_a, sizeB +- ds_a, more or less the same as before, but the uncertainty in sizeB given sizeA is much smaller sizeB = sizeA + delta_size +- dds. -Josh Kas

Hi Matt,

I tried that, before realizing it didn't really do anything. Performing a multi-dataset fit with two guessed variables, say size_A and sizeA_v_sizeB, is of course completely equivalent to just having the two sizes as guessed variables. And if size_A is set to some arbitrary (but reasonable) value, sizeA_v_sizeB is equivalent to just fitting size_B, and produces the "absolute" uncertainty again. This would be different if the fit were truly multi-dataset in the sense that we had parameters in common between samples, so that constraining the size of A had some effect on the fit for B. But the parameters that are in common, like S02, we constrained to a standard rather than refining through a multi-dataset fit.

I like your analogy to airline tickets...that is something like the situation we seem to have.

--Scott Calvin Sarah Lawrence College

...

Hi Scott,

Would it make sense to define a factor between the size of the two (or more) particles, say "sizeA_v_sizeB" and vary *that* in a fit of the two particles, keeping all the other things (k-weight, ranges, etc) the same for the two fits? That is, instead of asking "what is the size of particle A and what is the size of particle B?", ask "what is the size of particle A and how much bigger is particle B?". If you're observations are right, sizeA_v_sizeB should be statistically different from 1.

--Matt

PS: The price of airline tickets vary widely with many factors, but for any given flight, the price (starting "retail" price) of a first class ticket is always higher than a coach ticket. Of course, the coach ticket on some flights can easily be twice the price of a first class ticket on other flights. But everyone knows first class is always more expensive than coach. ;).

------------------------------

_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit

End of Ifeffit Digest, Vol 24, Issue 11 ***************************************