[R] Question about fitting power

From: Ana Quitério <ana.quiterio_at_ine.pt>
Date: Thu 26 Jan 2006 - 04:32:35 EST

 

Hi R users  

I'm trying to fit a model y=ax^b.

I know if I made ln(y)=ln(a)+bln(x) this is a linear regression.  

But I obtain differente results with nls() and lm()  

My commands are: nls(CV ~a*Est^b, data=limiares, start =list(a=100,b=0), trace = TRUE) for nonlinear regression

                        and :  lm(ln_CV~ln_Est, data=limiares) for linear
regression    

Nonlinear regression model: a=738.2238151 and b=-0.3951013  

Linear regression: Coefficients:

                                Estimate    Std. Error    t value
Pr(>|t|)

(Intercept) 7.8570224 0.0103680 757.8 <2e-16 ***

ln_Est -0.5279412 0.0008658 -609.8 <2e-16 ***    

I think it should be a=exp("(Intercept) ") = exp(7.8570224) = 2583.815 and b=ln_Est  

Probably I'm wrong, but why??    

Thanks in advance.  

Ana Quiterio    

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