Re: forcing fit parameters into a range

Posted by Michael Schmid on Oct 28, 2013; 12:53pm
URL: http://imagej.273.s1.nabble.com/forcing-fit-parameters-into-a-range-tp5005342p5005343.html

On Oct 28, 2013, at 12:57, Gelman, Laurent wrote:

> Hi everybody,
>
> Is it possible, when using the fit function, not only to give the initial values for the parameters, but also to force these to stay within a given range, e.g. never get negative or positive... ?

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Hi Laurent,

simply write your function such that it returns NaN (not a number) if it enters the forbidden range.
For this purpose, it's nice that the 'custom fit' function can also have a more complex form consisting of several statements.

Example:
  if (b < 0) y = NaN ; else y = a + b*x + c*x*x;

If you use one of the built-in fit functions, note that the fit procedure usually gets less efficient when converting it to a 'custom' function.  For the built-in functions, ImageJ splits off parameters that are mere factors and offsets and determines them by linear regression.  This is not done for custom functions.

Michael

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