Re: [ccp4bb]: Contour

["Aaron Oakley" <aoakley@receptor.pharm.uwa.edu.au>]

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So what do you chaps say about the significance
(in terms of sigma or otherwise) in AVERAGED
fo-fc maps?

Aaron


> > 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
> 


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