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

[Etienne Toffin]

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

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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