Hi JA, Oh dear, I seem to have gotten a rule named after me that I didn't even know I'd put down in print. As I recall, I got it from an offhand comment by Bruce, and he may have gotten it from Rossner and Krappe... :) Here's the deal in a nut-shell: The Nip you give is the Nyquist criterion. Or it's the Nyquist criterion +1, or the Nyquist criterion +2. In any case, the Nyquist criterion was developed for a signal. If I want to give you information in a periodic signal, how much information can I give you using a certain bandwidth and time? That's what Nyquist tells you. But there's no reason that Nature had to be so kind as to pack the maximum possible amount of information into an EXAFS signal. Some of the information may be redundant in some sense. So the true number of independent points might be somewhat less than Nip. The 2/3 is purely made up, as far as I know. It's like saying the fit is pretty close to the data when the EXAFS R-factor is less than 0.02. That 0.02 is arbitrary, but works as a rule of thumb. I myself don't always pay attention to the 2/3 that I have occasionally mentioned. I'm pretty sure I published stuff that's had more free parameters than that. In sum, I think it's bad to think that there is some magic number of free parameters, below which you're OK, and above which you're not. The fewer free parameters you can get away with, the better. And you should always give the k-space and r-space ranges in your publications, along with the number of free parameters, so that people can draw their own conclusions. If I see a published fit with 15 independent points by the Nyquist criterion and 14 free parameters, I'm not going to discount it completely, but I will be somewhat more skeptical than if it had only 4 free parameters, all else being equal. Hope that helps. --Scott Calvin Sarah Lawrence College At 02:29 PM 9/13/2006, Juan Antonio Maciá Agulló wrote:
Hi Ifeffit people,
I have a couple of short questions for you. I used the "Scott Calvin's rule" (number of variables < 2/3*Nip) to calculate the maximum number of allowed free parameters but I read that some people use the Nyquist theorem, which are the differences between them? and, which one is more correct?