[ccp4bb]: F, E and U notation...

[Kevin Cowtan <cowtan@ysbl.york.ac.uk>]

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Here's a question of notation on which I'd like to canvas opinion.

Is there a standard for whether |E| and |U| include a reflection multiplicity 
correction (i.e. epsilon)?

Or is there a good, brief, naming convention to distinguish magnitudes which 
do or do not include this correction? Should there be?

I guess the confusion arises from the definition of |E| by <|E|>=1. If the 
average is taken over all reflections (absent of not), then to obey the 
relationship the mutliplicity term is not needed. However, if the average is 
taken over non-absent reflections only, the data obeys the relationship only 
if the correction is included.

Just leafing though the documents I have to hand, Randy's and Garib's papers 
tend to include the correction, maybe to make the equations simpler. 
International tables B:2.1 doesn't.

The reason this arises is because the some of the generic programming I am 
doing in Clipper to operate on a whole load of data at once will need to 
behave differently for data which either includes or does not include a 
multiplicity correction. In which case, they need to be different types. 
Potentially there are 6 types of data:
F, E, U, corrected F, corrected E, corrected U
but how many of these will be used in practice is another question. Currently 
I'm assuming that the same types will hold F's, E's or U's, but that could be 
changed at the cost of multiplying the number of data classes and instances.

Kevin

-- 
Department of Chemistry, University of York, Heslington, York YO10 5DD