Oups, I sent the email by error, as I was still writing my reply…

Spencer,

Le 22-avr.-09 Ã  03:33, spencerg a Ã©crit :

>    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?
Yes, the first model is a special case of the second with eta1=0â¦
Could you give me more explanation about this method with likelihood  
ratio and 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?
>
I tried fitting the distribution using nls (perfect fitting). However,  
if I well understand 'anova' is used here to test alternative fittings  
(of alternative models) on a same distribution. Here I want to test  
the difference of parameter values between two different distributions  
and one same model (i.e. I want to test if eta1(TREATMENT) is  
different from eta1(CONTROL), and the same for other parameters…).

Do you have any other idea ?

Thanks,

Etienne

One short question about nls: are there any reason why nlm should be  
used rather than nls and vice-versa (nls results are quite more full  
than those of nlm)?



>    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 1Â°) values of etaD and cD parameters and 2Â 
>> °) 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
>> UniversitÃ© 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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>> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>

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