*** For details on how to be removed from this list visit the ***
*** CCP4 home page http://www.ccp4.ac.uk ***
Ulrich Genick schrieb:
*** For details on how to be removed from this list visit the ***
*** CCP4 home page http://www.ccp4.ac.uk ***
Hi,
a quick question for the data processing jocks.
I believe you ask a number of not strictly equivalent questions.
How much do systematic and random errors contribute to the
final sigmaF of a structure factor.
Weak reflections should be very little influenced by systematic error,
the errors of their intensities should be dominated by random error.
For strong reflections it should be the other way around.
Basically, I want to do some computation that involves the Fs from two
consecutive data sets taken
on the same crystal and in order to do
error propagation I need to know, if the sigmaF's for the same
reflection in the final merged data sets
are independent or correlated. My gut feeling is that the errors will be
mostly independent.
The sigmas of the datasets are not independant, rather they should be
similar (because the intensities should be the same, and strong
reflections have numerically high sigmaF's, so the sigma of a reflection
is correlated with its intensity).
I am positive somebody, (or probably a lot of people) know the answer to
this question and it is
probably written up somewhere in some old paper on scaling heavy metal
derivatives.
Here is the scenario, I take a crystal and collect the same data set
twice. Both data sets
have a decent redundancy of 3-5 and there is no noticeable
radiation-induced decay.
Will the sigmaFs of the two data sets overestimate the difference between
the two data sets (i.e. is there systematic error that is reproduced
between the data sets)
or will the sigmaF predict the difference between the two data sets
correctly (i.e. the error is random).
I am sure the answer will depend on the redundancy of the data sets, the
quality of the crystals, the
resolution the stability of the beam etc. etc. My question is what will
happen on average.
With good detectors and good data reduction procedures (and no other
systematic error present), the sigmas should be strongly correlated with
the differences between the intensities.
Just my 2 cents,
Kay
--
Kay Diederichs http://strucbio.biologie.uni-konstanz.de
email: [EMAIL PROTECTED] Tel +49 7531 88 4049 Fax 3183
Fachbereich Biologie, Universität Konstanz, Box M647, D-78457 Konstanz
