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? As always, thank you in advance Yours, Jesús
Jesus, On Mon, May 23, 2016 at 7:08 AM, Jesús Eduardo Vega Castillo < jevecas@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
Thank you very much Matt
2016-05-23 9:21 GMT-03:00 Matt Newville
Jesus,
On Mon, May 23, 2016 at 7:08 AM, Jesús Eduardo Vega Castillo < jevecas@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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participants (2)
-
Jesús Eduardo Vega Castillo -
Matt Newville