[R-sig-eco] Fixing parameters in GLM and Mixed Effects Models

[Jari Oksanen]

On 24/09/09 22:34 PM, "Hamish" <hwilman at ucsd.edu> wrote:

> 
> I want to force a =1 and b= -1, yielding:
> 
> 
> N ~ 1*Mass + (-1)*Temp + c*Precipitation + {d,e,f,g}*DIET
> 
I think you want to have

N ~ offset(Mass) + offset(-Temp) + Precipitation + DIET

Most *lm functions know offset. However, you must be very careful especially
with links other than identity. Moreover, you must be sure that this really
is what you're looking for: the above expression would be equivalent to
offset(Mass-Temp) and is Mass-Temp really something you think you want to
fix.
  
> 
> and then see what the values of the other parameters (c-g) turn out to be,
> as well as the model fit when using the forced parameters in conjunction
> with the free parameters.
> 
Cheers, Jari Oksanen