Order of fitting fixed effects

Dear All,

I am looking for some advice on determining the order in which fixed
effects should be fitted in an animal model.

I am currently carrying out association studies between SNPs and growth
in cows. My fixed effects include herd-year-season (HYS), covariate of
Age, and the SNP, and a random effect of 'Animal'. The model is
currently arranged in the following order -

Height ~ mu HYS Age SNP !r ANIMAL

I have found that if I change the order in which the fixed effects are
fitted, the results differ. After checking various papers, it appears
that some have fitted the SNP first, whereas others fit the SNP at the
end.

I would be very grateful for any clarification,

Kind Regards

Andrew

 
Received on Tue Jul 27 2009 - 16:22:25 EST