From: Joel Mackay <j.mackay@biochem.usyd.edu.au>
  To  : rasmb@alpha.bbri.org
  Date: Tue, 07 Sep 1999 12:33:25 +1000

significance?

Dear all,
I have a question about deciding which model describes one's data best. I
have recorded sedimentation equilibrium data at three speeds with three
different dilutions for a protein which undergoes some self-association. I
have been fitting the data in NONLIN. If i fit the data by fixing sigma to
the monomer mass, allowing delta y and lnA values to float, and permitting
a single association constant to float, i get the best fit with a
monomer-trimer model (both monomer-dimer and monomer-tetramer have worse
residuals and higher chi-squared etc according to NONLIN). If I instead
allow an extra equilibrium constant to float, and call the two associations
monomer-dimer and monomer-tetramer, i get a slightly lower chi-squared
(0.0138 vs 0.014 for the monomer-trimer model). My question is, how do i
decide if the extra complexity of the model is justified. I know there is a
thing called an F-test, and thought that might be appropriate. If so, how
does one apply it in this case? What do you all do in these situations? It
seems that the extra variable is pretty risky, but presumably a
sufficiantly large reduction in the chi-squared would justify its inclusion.
cheers and thanks in advance for any help,
Joel Mackay
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Dr Joel Mackay			ph +61-2-9351-3906
ARC Research Fellow			fax +61-2-9351-4726
Department of Biochemistry
University of Sydney
NSW 2006 Australia
http://www.biochem.usyd.edu.au/~joel/
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