[Ifeffit] doubt on number of variables

[Bruce Ravel]

On 05/26/2016 09:17 AM, Jesús Eduardo Vega Castillo wrote:
> As far as I understand the number of variables refers to those
> parameters which have been "guessed" in the fit and it is limited by the
> number of independent points.

Correct.

> If I have a group of variables but only set to guess a few of them while
> fixing the others, then fix some of the obtained values and guess other
> variables; I might include a large number of parameters in the resulting
> fit (even more than the independent points) without increasing the
> Number of variables of each individual fit. This also would keep reduced
> chi2 from increasing too much.
>
> Would this be cheating?

The answer, almost certainly, is "yes".

> The total number of parameters included is always restricted by the
> Number of independent points?

I'm going with this one ...

> or
> Can I include all the parameters I want as long as I set to guess no
> more that the number of independent points at the time?

... and I really don't like this one.


The thing you seem not to be considering is that parameters have 
correlations.  The correlations between parameters are an important part 
of the assessment of uncertainty.

If you artificially suppress correlations between parameters in the way 
that you are describing, it is unlikely that the fitted results and 
their uncertainties would be defensible.

While it is possible to probe correlations between parameters in a 
defensible manner by selectively setting and guessing parameters in a 
lengthy sequence of fits, your brief description does not give me 
confidence that that is what you are doing.

The trick to EXAFS analysis (and any non-linear minimization analysis) 
is to defensibly get what information you can from the data.  Rarely do 
we get everything we want.  In those situations where we aspire to  more 
than our data can provide, it's important to remember that something is 
better than nothing.

B

-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
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