[ccp4bb]: Phasing Phoiled by Phistats in Philadelphia. Pfui.

["Patrick J. Loll" <loll@pharm.med.upenn.edu>]

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I'm confused about output I obtain from phistats.  I'm attempting to
calculate the phase errors associated with various phasing procedures.
I've written a little program to calculate the quantity:

<|deltaPhi|> = the mean value over all reflxns of the absolute value of the
               difference between Phi1 and Phi2.  I calculate this for all
               reflxns and for acentrics only

In all cases, the DELF number obtained from phistats (described in the
output as "average difference in degrees" for acentric reflxns) is
substantially lower than the number I calculate independently (by, say, 20
degrees).

I've tried all the controls I can think of.  The most stringent control was
to write h, k, l, F, sigF, Phi1, & Phi2 into a single ASCII file, one line
per reflxn.  I made certain Phi1 and Phi1 both lay between 0 and 359 deg
(ie, not -180 to 180).   I can then use F2mtz to create an mtz file from
this & feed it into phistats.  This should cover most of the obvious
problems (eg, reflxns from different sources reduced to different asym
units).  The problem still persists.

I'm assuming DELF is equivalent to quantity I describe above.  Am I wrong?
I think it should mean the average absolute difference, rather than  the
"average difference," since the phase errors are uncorrelated, and over a
large dataset will average to zero.

Of course one might suspect the little program I've written; but I've
checked it every way I can imagine; and I think that even my meager coding
skills are up to the task of calculating a difference between two columns
of numbers and then averaging it...



Pat, Puzzled in Pennsylvania

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Patrick J. Loll, Ph. D.                             (215) 898-1294
Department of Pharmacology                     FAX: (215) 573-2236
University of Pennsylvania School of Medicine
3620 Hamilton Walk
Philadelphia, PA  19104-6084  USA

loll@pharm.med.upenn.edu
http://pharm.med.upenn.edu/Loll/loll_lab.html