Yu-Chuan, Matt and all,

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Yu-Chuan wrote:

At the beginning of the fitting, the S02(amp) are far away from 0.9 (just about 0.3). It's much different than Bruce's and Scott's examples. [...] Also, John have said that the S02 value should be around 0.9. That's why I am wondering if there is anything wrong with my data processing? Does anyone happen to have this kind of problem?

There are several possibilities. One is sample prep. [...] [...] Another is normalization.[...] Finally, look at the correlation between S02 and sigma2...if it is very high (say 0.95 or above) it may simply be coming up with low estimates for each.

Yu-Chuan didn't give details of sample prep or data collection, and I agree with Scott that these are most likely cause of low S02 factors (Scott mentioned problems with transmission measurements, and Grant mentioned how self-absorption can cause similarly low S02 factors for fluorescence measurements). I also agree with Scott that poor normalization will affect S02, though S02~0.3 would indicate an edge step off by a factor of ~3, which is a pretty big error for normalization. I doubt that is the culprit.

But: the correlation between S02 and sigma2 is NOT the cause of a value for S02 being low by a factor of 3. A high correlation means that a reasonably good fit could also be found by changing both S02 and sigma2 away from their best-fit value. This is not at all the same as saying that either parameter has the wrong best-fit value. In fact, the correlation between pair of variables says nothing about the trustworthiness of the best-fit values, only how much the uncertainties in the best-fit values depend on other variables. And, just to be clear, the reported uncertainties already take these correlation into account.

Yu-Chuan didn't actually say what the reported uncertainties in S02 were, but it seems that it was small enough so that S02~=0.3 could be distinguished from S02~=0.9.

--Matt

The degeneracy of the paths included in the fit were not mentioned, and could easily cause the S02 value to be off by a factor of 3. Especially, if you forget that feff has already taken the degeneracy of the path into account, and then you set some N-value to 2 or 3 or 4. I have seen others make this mistake before... Shelly