Dear Scott,
I have learned the Hamilton test. However, this method seems not suitable to distinguish our results because the Nvar are same for the two fittings (3 when S02 was fixed). So the b value should be zero and the calculator on the website http://www.danielsoper.com/statcalc/calculator.aspx?id=37 can not carry on.
In addition, I am not sure the value x. Is it the ratio of R-factors between two fits? I calculated the results using a=2.18 (Nidp and variables (Nvar) were 7.35 and 3), b=0.00001, and x=0.818 (the ratio of two R-factors) and got the Regularized lower incomplete beta function of 0.00000807. Does it mean something?
Cheers,
Shaofeng
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Shaofeng Wang, Ph.D of Geochemistry
Environmental Molecular Science Group
Institute of Applied Ecology, Chinese Academy of Sciences
Shenyang, 110016, China
wangshaofeng@iae.ac.cn
www.iae.cas.cn
From: Scott Calvin
Sent: Thursday, January 05, 2017 11:50 AM
To: XAFS Analysis using Ifeffit
Cc: Shaofeng Wang
Subject: Fwd: about sigma2 for exafs fitting
Shaofeng has given me permission to repost her question here on the ifeffit mailing list. It is quoted below my response.
Dear Shaofeng,
As Bruce and I said before, a sigma2 of 0.0007 A^2 is not impossible, although it indicates less disorder than is typically present. Your attached table does seem to show some improvement by using the model from the hydrogen-containing structure as compared to the arsenate. A more rigorous test for statistical improvement can be conducted using the Hamilton test (you mentioned you’ve consulted XAFS for Everyone; full details of the Hamilton test are given there).
It’s also encouraging that the uncertainties on your sigma2 determinations using the hydrogen-containing model are quite small; it appears that the fit is not getting confused by correlations even though it’s fitting both coordination number and sigma2, as that would generally also cause high uncertainties in the correlated parameters.
Is such a stiff sigma2 reasonable in this case? I have no idea. I just don’t know enough about this particular system; perhaps someone else on the list does. Oh, and one other question—was the data collected at room temperature? If it were collected at cryogenic temperatures, that would tend to reduce thermal disorder and thus lower sigma2’s.
Even if no one on the list has insight in to this particular system, anyone have good published examples of room-temperature systems with sigma2’s < 0.001? It might help Shaofeng with her referee…
Best,
Scott Calvin
Lehman College of the City University of New York
P. S. I certainly hope no reviewer is using the “typical values” I provide for parameters in XAFS for Everyone as rigid criteria for rejecting results! It is most certainly not the way I use them in the book. Some systems actually are atypical!
Begin forwarded message:
From: Shaofeng Wang