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CCP4BB  December 2008

CCP4BB December 2008

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Subject:

Re: R pim and Rmeans

From:

Frank von Delft <[log in to unmask]>

Reply-To:

Frank von Delft <[log in to unmask]>

Date:

Tue, 9 Dec 2008 22:27:00 +0000

Content-Type:

text/plain

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text/plain (65 lines)

Yes of course;  but the photons don't care what axis is being rotated 
about, they kill just as rapidly.
phx.


George M. Sheldrick wrote:
> When discussing this issue, perhaps we should not lose sight of 
> the fact that the statistics behind Rp.i.m. assume 'independent 
> observations'. Surely doing more than one rotation about the 
> same axis is likely to repeat the same systematic errors?
>
> George
>
> Prof. George M. Sheldrick FRS
> Dept. Structural Chemistry,
> University of Goettingen,
> Tammannstr. 4,
> D37077 Goettingen, Germany
> Tel. +49-551-39-3021 or -3068
> Fax. +49-551-39-22582
>
>
> On Tue, 9 Dec 2008, Frank von Delft wrote:
>
>   
>> Hi Manfred
>>
>>
>>     
>>> thanks a lot for your comments, since they raise some interesting
>>> points.
>>>
>>> R_pim should give the precision of the averaged measurement,
>>> hence the name. It will decrease with increasing data redundancy,
>>> obviously. The decrease will be proportional to the square root
>>> of the redundancy if only statistical errors or counting errors
>>> are present. If other things happen, such as for instance
>>> radiation damage, then you are introducing systematic errors,
>>> which will lead to either R_pim decreasing less than it should,
>>> or R_pim even increasing.
>>>
>>> This raises an important issue. As more and more images keep
>>> being added to a data set, could one decide at some point,
>>> when to add any further images? 
>>>       
>> This really is the point:  in these days of fast data collection, I assume
>> that most people collect more frames than necessary for completeness.  At
>> least, I always do.  So the question is no longer "is this data good enough"
>> -- that you can test quickly enough with downstream programs. 
>> Rather, it is, "how many of the frames that I have should I include", so that
>> you don't have to run the same combination of downstream programs for 20
>> combinations of frames.
>>
>> Radiation damage is the key, innit.  Sure, I can pat myself on the shoulder by
>> downweighting everything by 1/1-N -- so after 15 revolutions of tetragonal
>> crystal that'll give a brilliant Rpim, but the crystal will be a cinder and
>> the data presumably crap.
>>
>> But it's the intermediate zone (1-2x completeness) where I need help, but I
>> don't see how Rpim is discriminatory enough.
>>
>> phx.
>>
>>
>>     

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