[Ifeffit] uncertainties in "relative" values

[Matt Newville]

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. ;).