[Ifeffit] Re: A very quick question

[Scott Calvin]

Dave wrote:
>And....one other feature request for Bruce for Athena version 2.3.  It
would be nice if we could put constraints on each of the fitting variables.
 This would help prevent fits where the variables quickly spin off into
meaningless territory.  Of course, implemenation of this feature could get
complex given the number of variables to be fit.  An easier implemenation
of constraints would be a checkbox and a % deviation field that would let
the variables chosen float within that %deviation.
>>

Ifeffit (and thus Artemis) already has "max" and "min" features that allow
you to approximate this. For example, to require S02 to be between 0.60 and
1.00, you can do the following:

guess S02p = 0.80
def   S02p = max(0.60,min(1.00,S02p))

Admittedly that is a bit awkward, and Bruce could implement it as you
suggest in Artemis. My experience is, however, that this does not work very
well. A fit which likes to "spin off" tends to pin itself to one of the
edges of the allowed range anyway, and then the ifeffit routines for
working out uncertainties and correlations lose their ability to figure out
what is going on (the fitted parameter can become in some sense meaningless
to the fit). So I think if Bruce implements it that it would just be a
source of frustration.

You could use "restraints" instead, although that raises its own issues of
defensibility and how to choose the weighting function.

My preference is to find other ways to make the fit conform to the real
world, e.g. I include more paths, or more k-weights, or other constraints,
or values from a standard...

--Scott Calvin
Sarah Lawrence College