We know that our scaling models do not completely describe and compensate for all systematic errors, for various reasons including radiation damage (which is hard to model). This can be seen by scaling together data collected about different axes, where typically the merging statistics between different sweeps are worse than those within a sweep. Nevertheless, we would expect that averaging the somewhat disparate data should produce a more accurate estimate of the true intensity, provided that the underlying true intensities are really the same (isomorphism): merging statistics measure precision (internal agreement) rather than accuracy.
For measurement of anomalous differences, we should aim to measure the Bijvoet reflection pairs in as similar way as possible so that they are likely to have the same uncorrected systematic errors, most importantly to collect them close together in time, to minimise differences in radiation damage. However it is clearly also true that high multiplicity is extremely valuable, indeed essential for S-SAD, presumably just to average out errors, both random and systematic. Scala has an option to use matched pairs for estimation of anomalous differences (ANOMALOUS MATCH) but all the tests I've done using this option gave worse results than just merging everything. For this reason I haven't implemented the option in Aimless.
Phil
On 14 May 2013, at 12:07, Frank von Delft <[log in to unmask]> wrote:
> It's not about multiplicity, it's about scaling. See quote I sent earlier. phx.
>
>
> On 14/05/2013 10:40, Felix Frolow wrote:
>> I guess that in a standpoint to reduce errors it is easy to improve statistical errors by longer counting or by using multiple observations.
>> However the real enemy at the gate is a systematic error which require special skills and experience to detect and to eliminate.
>> I never understood why to measure not very good data trying to recover anomalous signal by improving statistics using very high redundancy instead of trying
>> to collect data which are perfect by minimising systematics errors and of course increasing counting time, but with minimum redundancy of only 2 ?
>> Like in good old times with 4 circle diffractometers and good scintillation counters that produced true counting statistics:
>> 10 counts - 30% precision
>> 100 counts - 10% precision
>> 1000 counts - 3% precision
>> 10000 counts - 1% precision
>> Canonising and worshipping redundancy looking for "true holy multiplicity" on my taste is counterproductive…..
>> My 2 NIS :-)
>> And of corse - one of the systematic errors is the radiation damage…...
>> FF
>>
>> Dr Felix Frolow
>> Professor of Structural Biology and Biotechnology, Department of Molecular Microbiology and Biotechnology
>> Tel Aviv University 69978, Israel
>>
>> Acta Crystallographica F, co-editor
>>
>> e-mail: [log in to unmask]
>> Tel: ++972-3640-8723
>> Fax: ++972-3640-9407
>> Cellular: 0547 459 608
>>
>> On May 14, 2013, at 11:19 , Tim Gruene <[log in to unmask]> wrote:
>>
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>>>
>>> Hi Frank,
>>>
>>> I would not call it 'axiomatic' but 'statistics' to reduce the
>>> (stochastic) error by several independent measurements. You can
>>> probably give any statistics textbook as a reference.
>>>
>>> In real life, though, you have to compromise with radiation damage,
>>> though. For references I recommend searching journals.iucr.org for
>>> 'Garman' as author. If you add 'radiation damaga' as keywords, the
>>> result reduces to 37 hits of choice.
>>>
>>> Best,
>>> Tim
>>>
>>> On 05/14/2013 06:50 AM, Frank von Delft wrote:
>>>> Hi, I'm meant to know this but I'm blanking, so I'll crowdsource
>>>> instead:
>>>>
>>>> Anybody know a (the) reference where it was showed that the best
>>>> SAD data is obtained by collecting multiple revolutions at
>>>> different crystal offsets (kappa settings)? It's axiomatic now (I
>>>> hope!), but I remember seeing someone actually show this. I
>>>> thought Sheldrick early tweens, but PubMed is not being useful.
>>>>
>>>> (Oh dear, this will unleash references from the 60s, won't it.)
>>>>
>>>> phx
>>>>
>>>
>>> - --
>>> - --
>>> Dr Tim Gruene
>>> Institut fuer anorganische Chemie
>>> Tammannstr. 4
>>> D-37077 Goettingen
>>>
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>>
>
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