[Ifeffit] Re: Artemis question

[Bruce Ravel]

On Thursday 22 January 2004 11:57 am, Matt Newville wrote:
> I agree with most of your answer to Dan about setting constraints
>
> and restraints, but I think I'd fail the 'quiz' you gave:
> > A word of caution.  Restraints can be dangerous.  Here is a quiz:  Why
> > is this definition of the restraint a bad idea?
> >
> >       set      scale = -100
> >       restrain amp   = abs(amp_guess - amp_target) * scale
>
> where you had
>
> >     guess    amp_guess  = 0.85
> >     set      amp_target = 0.85
> >     set      scale      = 100
> >     restrain amp        = abs(amp_guess - amp_target) * scale
>
>  
> as a recommended way to set just such a constraint. I believe
> there's no difference.  Am I missing something?

No... probably I am missing something.

It seems to me that if you use the absolute value to force the
difference to be a positive number, then set the scale to a negative
number, that would *reward* a large difference rather than *penalize*
it.

But perhaps I am confused in how the restraints are added to
chi-square.  I am under the impression that restraints are added as
written to chi-square.  This seems consistent with the language in
section 7.9 of the reference manual.  However, if the restraint is
squared before adding to chi-square then what I said about the
negative value of the scaling term is clearly wrong.

I should point out that I have not actually looked at the source
code...

B




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