Hi Matt, Thanks again for clearing up my confusion over the origin of the R factor for fits in R-space. I now have a different and unrelated question about the R factor. From looking through the ifeffit code, it appears that the R factor is defined as sum(data-fit)/sum(data). Is this correct? The reason that I am asking is that I would like to use Hamilton's test (Acta Cryst 1965, V18, p.502) to determine whether adding additional shells to a fit actually results in a better fit. Hamilton's test uses the "crystallographic R factor", which is sqrt(sum(data-fit)/sum(data)), so I would like to know whether or not to take the square root of the R-factor ratios in Hamilton's test. Thanks again for the help! Sincerely, Wayne --- Wayne Lukens Scientist Lawrence Berkeley National Laboratory email: wwlukens@lbl.gov phone: (510) 486-4305 FAX: (510) 486-5596