Stefano,
When it comes down to compare fits using different models, which is the relevant parameter? Chi square (and reduced) or R-factor? I have fits with smaller chi square and larger Rfactor and viceversa...
This is definitely a FAQ, so I put the answer at http://cars9.uchicago.edu/cgi-bin/ifeffit/faqwiz?req=show&file=faq08.013.htp
Is it possible that a rationale would be that when the models to compare have the same number of atoms these parameters go in parallel? And that when the number of atoms is different one must rely more on chi square rather than R-factor (or viceversa)?
I'm not sure I understnad this: I'd say the number of atoms or paths, or anything else that describes the complexity of the model, is NOT important. What's important is how well the specified variables cause the model to match the data. --Matt