Fixing parameters in GLM and Mixed Effects Models

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