[Ifeffit] Re: uncertainties in "relative" values (Josh Kas)

[joshua jason kas]

   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. ;).
>
>
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> End of Ifeffit Digest, Vol 24, Issue 11
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>