[R] An error in fitting a non linear regression

Petr PIKAL petr.pikal at precheza.cz
Wed Feb 25 09:58:23 CET 2009


Hi

r-help-bounces at r-project.org napsal dne 24.02.2009 12:41:50:

> 
> Hi Peter,
> 
> You are totally right and it was a miscalculating and misunderstanding 
from
> me.
> Regarding the R-squared calculation of non linear model (question 2), is
> there any way to do that?

I am not an expert statistician, Douglas Bates is so see his answer to 
similar question few years ago

http://tolstoy.newcastle.edu.au/R/help/02b/0482.html

Regards
Petr

> 
> Regards
> Saeed 
> 
> Petr Pikal wrote:
> > 
> > Hi
> > 
> > r-help-bounces at r-project.org napsal dne 24.02.2009 11:31:22:
> > 
> >> 
> >> Hi,
> >> 
> >> Thank you for the reply and suggestions.
> >> 
> >> I have two questions?
> >> 1) If I want to use log, it seems that I have to take log from both 
> > sides of
> >> the model which will lead to lm(log(q)~log(-depth)). What is 
> > tehdifference
> >> between this syntax and lm(log(q) ~ I(-depth))?
> > 
> > 
> > If you have 
> > 
> > y = a*exp(-b*x) then log of this equation is
> > 
> > log(y) = log(a) - b * x
> > 
> > at least I was told that by my teacher back at school some decades ago 

> > that log(exp(x)) = x.
> > 
> > You can prove it by
> > 
> > log(exp(whatever))
> > 
> > interactively in R
> > 
> > Regards
> > Petr
> > 
> >> 
> >> 2) How can I calculate the R-squared of a fitted non linear model?
> >> 
> >> Regards
> >> Saeed
> >> 
> >> 
> >> Christian Ritz-3 wrote:
> >> > 
> >> > Hi Saeed,
> >> > 
> >> > one approach is to try out several initial value combinations for a 

> > and b.
> >> > 
> >> > It often helps to find initial values of the same order of 
magnitude 
> > and
> >> > of the same sign
> >> > as the final estimates.
> >> > 
> >> > To get such initial values, you could linearize the model:
> >> > 
> >> > lm(log(q) ~ I(-depth))
> >> > 
> >> > 
> >> > and supply the estimated coefficients from the linear regression as
> >> > starting values:
> >> > 
> >> > nreg <- nls(q ~ a*exp(-b*depth), start = list(a = 0.76168, b = 
> > -0.08484))
> >> > summary(nreg)
> >> > 
> >> > 
> >> > Christian
> >> > 
> >> > ______________________________________________
> >> > R-help at r-project.org mailing list
> >> > https://stat.ethz.ch/mailman/listinfo/r-help
> >> > PLEASE do read the posting guide
> >> > http://www.R-project.org/posting-guide.html
> >> > and provide commented, minimal, self-contained, reproducible code.
> >> > 
> >> > 
> >> 
> >> -- 
> >> View this message in context: 
> > http://www.nabble.com/An-error-in-fitting-a-non-
> >> linear-regression-tp22118160p22179525.html
> >> Sent from the R help mailing list archive at Nabble.com.
> >> 
> >> ______________________________________________
> >> R-help at r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide 
> > http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> > 
> > ______________________________________________
> > R-help at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide
> > http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
> > 
> > 
> 
> -- 
> View this message in context: 
http://www.nabble.com/An-error-in-fitting-a-non-
> linear-regression-tp22118160p22180225.html
> Sent from the R help mailing list archive at Nabble.com.
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> 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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