# How to select best model out of linear, quadratic, and a model involving an exponential? Are the models nested?

I'm examining the growth rate of species en function of ion carbonate. For each specie, I have different measured of ion carbonate, it can be in different temperature or pH.

For each measure, I'm doing graphics:

• x - ion carbonate;
• y - growth rate.

I know that for each measure I can have one of this models:

1. y=ax+b
2. y=ax^2+bx +c
3. y=a*(1-exp(-x/b)) + c

so I have to choose which one to select. I'm doing this with AIC. But AIC is only for nested models, is my 3 model is nested? 1 and 2 are nested I think.

If not, how can I select what is the "best" model for my data?

• And I believe the specific justification for saying it's nested is that you can create 1 from 2 by restricting parameters: Fix 2's a=0 and you get 1. – Wayne Oct 6 '11 at 18:06