[R] How to compare parameters of non linear fitting curves

[spencerg]

      Is your first model a special case of the second with eta1 = 0?  
If yes, what about using 2*log(likelihood ratio) being approximately 
chi-square? 


      Can you recast the problem to use "nls"?  If yes, might the 'nls' 
methods for 'anova' or 'profile' give you what you want? 


      Hope this helps,
      Spencer

Etienne Toffin wrote:
> Hi,
>
> I'm using a non linear model to fit experimental survival curves.
>
> This model describes the fraction of "still active" experiments as a 
> function of time t as follows:
> f(t)=(1+exp(-etaD*cD)) / (1+exp(etaD(t-cD)))
>
> Moreover, when experiments are still active, they may change of state 
> (from 0 to 1). But they may fall inactive before changing their state 
> (their state still equals 0). The survival curve of state may also be 
> fitted with the following model:
> f(A)=(1+exp(-eta1*c1)) / (1+exp(eta1(t-c1))) * (1+exp(-etaD*cD)) / 
> (1+exp(etaD(t-cD)))
>
> I estimate with nlm 1°) values of etaD and cD parameters and 2°) 
> inject them as constant in the function to be minimized by nlm to 
> estimate values of eta1 and c1.
>
> I perform these estimations for two different experimental conditions 
> that both have their values of etaD,eta1, cD and c1.
>
> I would like to know if there is any statistical method to compare the 
> estimated values of parameters of the two distributions ? And wether 
> it's the case, how to perform it in R ?
>
> Hope I'm clear enough for getting help,
>
> Etienne
>
> -------------------------------------------------------------------
> Etienne Toffin, PhD Student
> Unit of Social Ecology
> Université Libre de Bruxelles, CP 231
> Boulevard du Triomphe
> B-1050 Brussels
> Belgium
>
> Tel: +32(0)2/650.55.30
> Fax: +32(0)2/650.57.67
> http://www.ulb.ac.be/sciences/use/toffin.html
>
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