[Ifeffit] Re: sigma squared

[Matt Newville]

Karla,

> > I have a couple of questions.  First, in Artemis, when I click on
> > the "x" buttons to select my k- and r-ranges, the window never seems
> > to correspond to where I clicked.  Is this a bug?
> 
> Probably not.  The default in Artemis is use a Kaiser-Bessel window.
> The various strange words to describe FT windows ("Kaiser-Bessel",
> "Hanning", "Welch", and so on) refer to different functional forms for
> ...

I don't understand "the window never seems to correspond to
where I clicked".  What should happen is this (in order):
  - you click on the [x] box to the right of a parameter
  - you click on the plot window
  - the 'x' (E, k, R) value of the point you clicked on 
    gets written into the entry for the parameter value 
    (to the left of the box).
  - you re-generate the plot to have this selected value 
    take effect.

Is that not what happens?  Are you saying that the wrong 'x'
value is getting written into the parameter entry or that the
window looks different than you think it should?

For the negative sigma2 values, I agree with Bruce: it's not a
bug, exactly.  More generally, it's not trivial for the fitting
procedure to know that some variables or even Path Parameters
have "unreasonable values" unless a) the program enforces how
you model the data (which leads to loss of flexibility) or b)  
the user explicitly says what "reasonable values" are, either
using rigid math expressions or the restraint mechanism.

--Matt