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st: RE: nonlinear-leastsquare


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: nonlinear-leastsquare
Date   Mon, 1 Dec 2003 11:24:25 -0000

Andreas Aschbacher
 
> I have two columns in ascii-sheet :
> x-col        y-col
> 1              34
> 2              45
> 3              44
> ... and so on         /experiment in radioactive measurement /
> 
> and the following program in a do-file:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> capture program drop nlexample
> program nlexample
> version 8
> if "`1'" == "?" {
> global S_1 "a1 a2 a3 a4 a5 a6"

... 

> I reached convergence and good values for a1,a2,...with 
> "right"start-values
> above after long 'trying' ......

> my questions::
> 1.) which parameter is the typical Marquardt-Parameter ?

Pass. 

> 2.) although in reference-book:what does query-call and what does
> calculation-call mean ?

Think of the query call as initialising the problem and 
the calculation calls as doing the numerics and statistics. 

> 3.)after calling the program above with >do name< and after 
> getting my
> results I print: predict myvar
>     then I  have  with the command :twoway line myvar x a passable
> graph-fit,is this the correct way ?

This plots predicted against predictor. 

Given that curve, 

. twoway mspline myvar x 

might look better. 

You might also want to add the data points, say by 

. twoway mspline myvar x || scatter y x 

Alternatively, 

. ssc desc regplot 
. ssc inst regplot 
. <nl fit> 
. regplot y 

Nick 
n.j.cox@durham.ac.uk 

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