[Ifeffit] Re: Artemis question

[Matt Newville]

Hi Bruce,

> 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.

Nope, the idea is to make the value of the restraint 0.  Negative
and positive errors in the value are penalized equally.

> 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.

A restraint is an additional element in the vector to be minimized.
chi-square is the sum of squares of the elements of this vector.

I guess I should fix the document!

--Matt