I think that with speculation and head-scratching one cannot answer all questions. A very helpful possibility is simulation: you could simulate datasets with different twin fractions, detwin them, and look at the maps (or characterize the data otherwise, like doing substructure solution). This offers a way to stepwise deteriorate the data, and to test with different completeness values.
Actually, artifically twinned data are already available - James Holton posted them.
HTH,
Kay
On Thu, 12 May 2016 11:38:52 +0100, Eleanor Dodson <[log in to unmask]> wrote:
>But Jacob, did it work so badly? You said the *actua*l peak height for your
>Dano map had increased - although the Peak/Sigma had gone down.. But there
>are all sorts of reasons for that - which would be hard to analyse - you
>would need to look at a list of spurious peaks I suppose,
>
>Eleanor
>
>
>On 11 May 2016 at 22:57, Keller, Jacob <[log in to unmask]> wrote:
>
>> I know both from Eleanor and from looking at the completeness of my
>> datasets before/after detwinning that Detwin simply omits incomplete
>> twin-pairs. I wonder how to assess the gains from implementation of what
>> you suggest?
>>
>> The question which keeps nagging at me is simply why detwinning works so
>> badly--maybe your suggestion would improve it? I have some possibilities
>> for other reasons why detwinning does not work well, but they don't seem
>> powerful enough to overcome the apparent much-worseness of twinned data.
>>
>> Again, it seems to me that the way to beat twinning is to model it from
>> start to finish.
>>
>> JPK
>>
>> -----Original Message-----
>> From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Kay
>> Diederichs
>> Sent: Wednesday, May 11, 2016 5:30 PM
>> To: [log in to unmask]
>> Subject: Re: [ccp4bb] Dano--Sign Convention
>>
>> Jacob,
>>
>> my intuition was also wrong - the math is such that the "calculating the
>> anomalous difference" and "detwin" operations can be exchanged. In other
>> words, what I suggested to be possibly different options a) and b) are
>> actually mathematically equivalent. Thus, none of them has an advantage
>> over the other.
>> It remains the option to decide what to do with DANOtw(h1) if DANOtw(h2)
>> is unknown. I think that the decision should be done in a probabilistic
>> way: if the error from omitting DANOtrue(h1) (or equivalently, setting
>> it to zero) is likely higher than the error from just taking DANOtw(h1)
>> as DANOtrue(h1), then one should do the latter.
>> This should be the case when the magnitude of DANOtw(h1) is high, and it
>> should also be the case when the twinning fraction tf is low (so a test
>> combining both criteria would be to compare DANOtw(h1)/tf against
>> <DANO>/0.5).
>> I don't know if CCP4 detwin does anything like this, but it could be
>> implemented - it might improve the anom Patterson and the maps from the
>> detwinned data.
>>
>> best,
>>
>> Kay
>>
>>
>> Am 11.05.16 um 21:18 schrieb Keller, Jacob:
>> >> in your other posting, you gave the example (using the notation of the
>> > detwin documentation):
>> >
>> > DANOTw(h1) = 52
>> > DANOTw(h2) = 28
>> >
>> > and if you use the formulas that you cite below, you get the true values
>> > back:
>> >
>> > DANOtrue(h1) = (0.6*52 - 0.4*28)/(1-0.8) = 100
>> > DANOtrue(h2) = (0.6*28 - 0.4*52)/(1-0.8) = -20
>> >
>> > So then, what's problematic?
>> >
>> > ======================
>> >
>> > Nothing--I think you're right, and my intuition was wrong. Thanks for
>> correcting my mis-statement.
>> >
>> > What would come of detwinning Dano rather than I+/-? I wonder, like you,
>> whether anything would be gained or lost?
>> >
>> > JPK
>> >
>>
>>
>
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