[R] Question abour nonlinear regressions

[Benoit Boulinguiez]

Hi all,

I try to use R to compare models of adsorption onto activated carbon
materials.
Three models are compared:

(1)Langmuir
qe~a*b*Ce/(1+b*Ce)

(2)Freundlich
qe=b*Ce^a

(3)Langmuir-Freundlich
qe~a*b*Ce^z/(1+b*Ce^z)

Experimentally I measure Ce and qe, thus I obtain a dozen of paired values
(Ce,qe) which represents an isotherm of adsorption.
I use the nls() function to assess the parameters of the models and
confint() to get their confidence intervals.

To estimate the fit, I plot the residuals for each models and compare the
SSR values. However, as the model (3) has 3 parameters where models (1) and
(2) have 2, I use the standard deviation to estimate models' fitting each
other.

May I use a sort of R2 determined with the cor() function between the data
and the fitted values, to compare the models as well?


Regards/Cordialement


Benoit Boulinguiez