I also think Rpim, as commonly defined, is not directly useful for measuring the anomalous signal. Commenting on its use, though, I find it quite useful to judge whether one is adding useful data to already existing ones. In a multiple crystals context I always look at both Rmeas and Rpim. Suppose a given dataset shows some Rmeas and Rpim. Now add another dataset to the first one, in the hope to increase completeness and redundancy. This, of course, will depend on how isomorphic were the crystals from which the two datasets were collected. If the Rmeas stays stationary, or increases
just a little, and Rpim decreases, this means thatthe crystals had a good degree of isomorphism and that scaled data will be more precise. This is what one wished for, when collecting from multiple crystals. On the other hand, an increase in Rpim definitely points at something wrong with the addition of the second dataset, quite certainly some form of non-isomorphism. J Dr James Foadi PhD Membrane Protein Laboratory Diamond Light Source Ltd. Diamond House Harwell Science and Innovation Campus Didcot Oxfordshire OX11 0DE United Kingdom office email: james.fo...@diamond.ac.uk alternative email: j.fo...@imperial.ac.uk personal web page: http://www.jfoadi.me.uk ________________________________ From: Jim Pflugrath <jim.pflugr...@rigaku.com> To: CCP4BB@JISCMAIL.AC.UK Sent: Friday, 2 November 2012, 19:24 Subject: Re: [ccp4bb] Rpim and how its related to anomalous signal In my opinion Rpim is not related directly to anomalous signal, so perhaps that is why there is some confusion. Also I think some folks confused Rpim with Rrim. The latter is also called Rmeas. But once again, these are not related directly to anomalous signal. I do not find Rpim very useful for anything since when the data has high multiplicity Rpim gets very low, but as Michael Blum told me, "Precision does not trump accuracy." I would personally rather have accurate data than highly redundant but inaccurate data. I like to look at the deltaF / sigma(deltaF) value reported by SHELXC and other programs, where deltaF is ||F+| - |F-||. This might also be called <d"/sig>. There are other things to look at as well. Jim From: CCP4 bulletin board [CCP4BB@JISCMAIL.AC.UK] on behalf of Vijayakumar.B [vijaybioscie...@gmail.com] Sent: Tuesday, September 18, 2012 4:36 AM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] Rpim and how its related to anomalous signal Dear CCP4BB users, I am very much interested to work in Anomalous scattering technique for macromolecular structure determination. I have already gone through some literature in which they have explained about a parameter called “Rpim”. I am little bit confused about Rpim values. Can anyone tell me about the use of this parameter in structure determination by anomalous techniques and how Rpim is related to anomalous signal? Thanks in advance With kind regards B. Vijayakumar