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
