[ccp4bb]: Contour

["Leslie A." <andrew@mrc-lmb.cam.ac.uk>]

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> so you suggest that we should go back to the dark
 > ages and put a water in every 3 sigma Fo-Fc peak ?
 > (that's what people may be tempted to do after
 > reading this)
 > 
 > and again: "5 sigma" is a meaningless concept in an
 > Fo-Fc map - you must talk in absolute terms. we're
 > talking about electrons here, not statistics. saying
 > that 'N sigma' is a significant feature no matter what
 > is like saying that a beer should cost one currency
 > unit in every country, i.e. a pound in the uk and a
 > lira in italy. you need to put them on an absolute
 > scale (e.g., euros ;-)
 > 
 > --gerard

I made no such suggestion. I clearly stated that one might not be able
to interpret such features. 

On reading your own original statement, people might be tempted to
believe that a 5 sigma peak in their final difference map was not
significant. I assert again that in the case under discussion
(difference maps for a macromolecular structure) this is not the case.
I also assert that for macromolecular structures, it is actually NOT
necessary to calculate the difference maps on an absolute scale, as
you are suggesting, simply because there are always sufficient errors
in the model to ensure that a 5 sigma peak is not just "noise".

  Andrew Leslie