[R] nls segmented model with unknown joint points

[andy]

Thank you very much Dr. Venables, I'll give this a try.
Regards-
andy

On Sun, 2005-04-17 at 13:36 +1000, Bill.Venables at csiro.au wrote:
> This is how I'd write the formula for use with nls/nlme: 
> 
> y ~ b41*(x - 1) + b42*(x^2 - 1) + 
>  ifelse((a41 - x) >= 0, b43*(a41 - x)^2, 0) +
>  ifelse((a42 - x) >= 0, b44*(a42 - x)^2, 0)
> 
> This is a direct translation from your funny foreign-looking code below
> that probably makes it clear what's going on.  A more swish R form might
> be
> 
> y ~ b41*(x - 1) + b42*(x^2 - 1) + 
> 	b43*pmax(a41 - x, 0)^2 + b44*pmax(a42 - x, 0)^2
>  
> You mention nlm, too.  Here you would use a function rather than a
> formula, but the idea is the same.
> 
> V.
> 
> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of andy
> Sent: Sunday, 17 April 2005 1:09 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] nls segmented model with unknown joint points
> 
> 
> Hello,
> 
> I am interested in fitting a segmented model with unknown joint points
> in nls and perhaps eventually in nlme.  I can fit this model in sas (see
> below, joint points to be estimated are a41 and a41), but am unsure how
> to specify this in the nlm function.  I would really appreciate any
> suggestions or example code.  Thanks a lot. -andy  
> 
> proc nlin data=Stems.Trees;
>  params  b41=-3 b42=1.5 b43=-1.5 b44=50 a41=0.75 a42=0.1;
> 
>  term1 = (b41*(x - 1) + b42*(x**2 -1));
> 
>  if (a41 - x) >= 0 then 
> 	term2 = (b43*(a41 - x)**2);
>  else
> 	term2 = 0; 
> 
>  if (a42 - x) >=0 then
> 	term3 = (b44*(a42 - x)**2); 
>  else 
> 	term3 = 0;
> 
>  model y = term1+term2+term3;
> run;
> 
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