Groups > comp.graphics.apps.gnuplot > #322

Re: Wrong fit direction / zrange / unreadable graph file

From	Hans-Bernhard Bröker <HBBroeker@t-online.de>
Newsgroups	comp.graphics.apps.gnuplot
Subject	Re: Wrong fit direction / zrange / unreadable graph file
Date	2011-05-19 01:49 +0200
Message-ID	<93j47sF2m1U1@mid.dfncis.de> (permalink)
References	<01ac512a-8031-41f0-a899-0684927f6c32@n10g2000yqf.googlegroups.com> <iqqpas$cec$1@gwdu112.gwdg.de> <93c9dhFmvmU1@mid.individual.net> <iqqs4o$fkv$1@gwdu112.gwdg.de> <93cqpmF1a3U1@mid.individual.net>

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On 16.05.2011 16:31, Ingo Thies wrote:

> Strange. I see no reason for this since n is merely an additional offset
> which could, depending on the function, even be negative. And the DNLS1E
> routine from SLATEC doesn't have any problems here. How is fitting done
> in gnuplot in particular?

Marquardt-Levenberg via QR decomposition.

> as does gnuplot with non-zero initial n. Maybe that routine has some
> singular matrix trap which gnuplot doesn't have.

The problem is not actually in the computations with matrices.  It's in 
how gnuplot computes the derivatives of the model WRT the parameters at 
each data point (a.k.a. the "Jacobian" matrix).  That derivative 
computation simply doesn't work at zero.

The second part of the problem is that we don't normalize parameters to 
a common scale, so parameters of wildly different magnitude cause the 
fit to grind to a halt.

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Thread

Wrong fit direction / zrange / unreadable graph file Katharina Miwege <k.miwege@googlemail.com> - 2011-05-15 23:59 -0700
  Re: Wrong fit direction / zrange / unreadable graph file Christoph Bersch <usenet@bersch.net> - 2011-05-16 11:04 +0200
    Re: Wrong fit direction / zrange / unreadable graph file Ingo Thies <ingo.thies@gmx.de> - 2011-05-16 11:34 +0200
      Re: Wrong fit direction / zrange / unreadable graph file Christoph Bersch <usenet@bersch.net> - 2011-05-16 11:52 +0200
        Re: Wrong fit direction / zrange / unreadable graph file Richard Owlett <rowlett@pcnetinc.com> - 2011-05-16 06:25 -0500
          Re: Wrong fit direction / zrange / unreadable graph file Christoph Bersch <usenet@bersch.net> - 2011-05-16 14:44 +0200
        Re: Wrong fit direction / zrange / unreadable graph file Ingo Thies <ingo.thies@gmx.de> - 2011-05-16 16:31 +0200
          Re: Wrong fit direction / zrange / unreadable graph file Katharina Miwege <k.miwege@googlemail.com> - 2011-05-16 09:01 -0700
            Re: Wrong fit direction / zrange / unreadable graph file Christoph Bersch <usenet@bersch.net> - 2011-05-17 08:46 +0200
              Re: Wrong fit direction / zrange / unreadable graph file Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2011-05-19 01:34 +0200
          Re: Wrong fit direction / zrange / unreadable graph file Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2011-05-19 01:49 +0200
            Re: Wrong fit direction / zrange / unreadable graph file Ingo Thies <ingo.thies@gmx.de> - 2011-05-19 11:07 +0200
              Re: Wrong fit direction / zrange / unreadable graph file Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2011-05-19 14:02 +0200
      Re: Wrong fit direction / zrange / unreadable graph file Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2011-05-19 01:39 +0200
        Re: Wrong fit direction / zrange / unreadable graph file Ingo Thies <ingo.thies@gmx.de> - 2011-05-19 10:58 +0200

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