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. ;).