[Ifeffit] Consultation on Reduced Chi2

[Jesús Eduardo Vega Castillo]

Thank you very much Matt

2016-05-23 9:21 GMT-03:00 Matt Newville <newville en cars.uchicago.edu>:

> Jesus,
>
>
>
> On Mon, May 23, 2016 at 7:08 AM, Jesús Eduardo Vega Castillo <
> jevecas en gmail.com> wrote:
>
>> Dear list,
>>
>> I am back with a new consultation on EXAFS fitting within Artemis.
>>
>> I have made a fit and obtained a reduced Chi2 value of 391 and R-factor
>> of 0.014 using 9 variables. Then I have added two more paths increasing the
>> number of variables up to 12 and then I obtained a reduced Chi2 of 3039 and
>> R-factor of 0.007.
>>
>> I am a little surprised by the huge increase of reduced Chi2 while
>> R-factor decreases down to half.
>>
>> What could be the cause of this large increase?
>> Does it mean that it is not worth to add these new paths?
>>
>>
> Reduced chi-square is scaled by dividing by the number of free parameters,
> (Nidp - Nvarys).  Increasing the Nvarys from 9 to 12 without also
> increasing Nidp (by adding data k or R range), could definitely make
> reduced chi-square increase even if the R-factor and chi-square decrease.
>
> One of the main uses of reduced chi-square is to determine if additional
> variables are improving the fit well enough to justify their inclusion in
> the model.
>
> --Matt
>
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