[R] nls segmented model with unknown joint points

From: andy <afinley_at_gis.umn.edu>
Date: Sun 17 Apr 2005 - 13:08:48 EST


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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