[R] An error in fitting a non linear regression

[Saeed Ahmadi]

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?

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.
> 
> 

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