Spencer,
Le 22-avr.-09 à 03:33, spencerg a écrit :
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=0…
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
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 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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