 Dear R-sig-ecology group,
 
I have a question about trend surface analysis TSA  and the way to simplify polynomials. 
I searched in literature but could not find any clear indication. 
Assuming a trend surface based on a polynomial of 2nd order like this:
Model A:   Z ~  β0 +  β1 x  +  β2 y +  β3 xy +  β4 x^2 +  β5 y^2 
 
Can I fit a model B  with β4 = 0  and compare the two models A and B based on their AIC?  Is it correct to do this?  It was done in several papers using TSA, but I’ll really appreciate to know if we can actually do such a simplification with polynomials.
 
Cheers, 
Maryline


Maryline PIOZ, Ph.D.

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