[R] A problem with anova()

[Peter Dalgaard]

Guru S wrote:
>   
> Hi,
>
> I fitted tree growth data with Chapman-Richards growth function using nls. 
>
> summary(fit.nls) 
>
> Formula: HEIGHT ~ A * (1 - exp(-B * AGE))^C 
>
> Parameters: 
>
>   Estimate Std. Error t value Pr(>|t|)    
> A 29.007627  0.270485  107.24  <2e-16 ***
> B  0.030813  0.001095  28.13  <2e-16 ***
> C  1.849405  0.068659  26.94  <2e-16 ***
> ---
> Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 
>
> Residual standard error: 1.879 on 713 degrees of freedom
>
> Algorithm "port", convergence message: relative convergence (4) 
>
> When I try to run the anova() function I get this:
>
> anova(fit.nls)
> Error in anova.nls(fit.nls) : anova is only defined for sequences of "nls" objects
>
> Could you tell me what the problem is?
>
>   
It's what the message says: You can't run anova() on one model, only 
compare several, as in
anoval(fit.nls, fit2.nls). 

It is not clear what anova(fit.nls) should do in the nonlinear case , 
since you cannot in general remove parameters successively from the 
model (notice, for your own model, that C is meaningless if B=0 and both 
B and C are meaningless if A=0).

-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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