[R] Nonlinear Least Squares nls() programming help

Dieter Menne dieter.menne at menne-biomed.de
Mon Jul 13 16:44:35 CEST 2009




MathZero wrote:
> 
> Hi, I am trying to use the nls() function to closely approximate a vector
> of values, colC and I'm running into trouble.  I am not sure how if I am
> asking the program to do what I think its doing, because the same
> minimization in Excel's Solver does not run into problems.  
> 
> 

Fitting a function is an approximation, trying to find a minimum. Think of
frozen mountain lake surrounded by mountains. Excel's Solver will report the
highest tip of the snowflake on the lake, if it finds it. nls will find out
that the lake is essentially flat compare to the surrounding and tell you
this fact in unkind word.

In your case, I assume there is an (additional?) problem that the solution
might be non-identifyable. It could required additional constraints, for
example that one coefficient is always larger than the other.

Dieter


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