I had learned that Rsym compared symmetry-related reflections within an image or film (in the context of a rotation exposure), and Rmerge compared symmetry-related reflections on different images or films with one another.
Is that wrong? Bill Manfred S. Weiss wrote: > Dear Mohd and all others, > > Well, I guess it is time again to define and talk about R-factors. > > The term R_sym goes back to the times, when X-ray data were > recorded by precession photography on film. Except for the central > cone, each reflection was observed only once and R_sym described > the agreement strictly between symetry-related reflections, hence > R_sym. > > In diffractometer times, when reflections were measured one by one, > only for a subset of reflections (typically a plane in reciprocal > space) were symmetry-related reflections measured two times. R_sym > became R_int, where int stands for internal agreement. These > additional reflections were just measured to calculate R_int, > later on they were discarded. > > Nowadays, where a diffraction data set typically consists of dozens > or hundreds of images recorded from some sort of an area detector, > multiple measurements of the same reflection AND symmetry-related > reflections are merged together to calculate the mean intensity > for a given reflection. Hence, the agreement factor becomes R_merge > or merging R-factor. > > As you can see, R_merge is more general than R_sym, and is (as Kay > pointed out) the preferred term. > > However, when talking about R-factor I can never refrain from > mentioning that R_merge should actually NEVER EVER be used, > because it is inherently flawed. As the redundancy or the > multiplicity of the data increases, R_merge will also increase, > although the mean intensity will be more precisely determined. > As was postulated by Kay and myself about 10 years ago, R_merge > should be replaced by a redundancy-independent merging R-factor > (termed R_rim or R_meas). Unfortunately, only SCALA and XDS > produce this R-factor, SCALEPACK does not (not yet, I hope --> > pun to Dallas). If you want to calculate R_rim or R_meas based > on scaled but unmerged data, I have my own program, which you > can download from my web site, as does Kay. > > I hope this clarifies things. > > Cheers, Manfred. > > ******************************************************************** > * * > * Dr. Manfred S. Weiss * > * * > * Team Leader * > * * > * EMBL Hamburg Outstation Fon: +49-40-89902-170 * > * c/o DESY, Notkestr. 85 Fax: +49-40-89902-149 * > * D-22603 Hamburg Email: [EMAIL PROTECTED] * > * GERMANY Web: www.embl-hamburg.de/~msweiss/ * > * * > ******************************************************************** > > > On Fri, 18 Jan 2008, Salameh, Mohd A., Ph.D. wrote: > >> Hi everybody! >> I will appreciate it if anybody can clarify to me the differences >> between Rmerge and Rsym. Many thanks, M >> >> **************************************************** >> Mohammed A. Salameh, Ph.D. >> Mayo Clinic Cancer Center >> Griffin Cancer Research Building >> 4500 San Pablo Road >> Jacksonville, FL 32224 >> Tel:(904) 953-0046 >> Fax:(904) 953-0277 >> [EMAIL PROTECTED] >> **************************************************** >> >> >> >
