[R-sig-ME] LME and nonlinearity?

[Balázs Lestár]

Dear All,

I have a mixed model (LME), but one of my explanatory variables is not linearly related to the dependent variable.

 1.)     Somebody told me, to make a 2 or 3 level factor from the continuous variable. (I wouldn't prefer this)

  2.)     I saw in some statistical books that in these cases, I have to use in the model the quadratic term of the variable. (but the AIC is much greater than with the  factorized variable)

OR

 Is that possible, to use a poly() function in the lme? (this model seems to be the best, based on AIC).
 

I'm a bit confused, 'cause the LME supposes linear relation between variables. Isn't it right?

 3.)        Do I need a non-linear model?

Which solution is the best?


Regards,
Balazs