Hi Marcus I don't use CC(1/2) as a criterion for cut-off though I do keep an eye on it. It's not clear to me how you apply the CC(1/2) criterion when the data are incomplete (as they invariably are). Also there seems to be some debate whether the cut-off should be at CC = 0.5 or much lower, e.g. 0.125 representing the level of statistical significance, as you suggest. In the end I suspect it makes little difference.
Instead I use the average (I / sigma(I)) for all reflections, i.e. including unmeasured for which I take (I / sigma(I)) = 0, as the cut-off criterion with a cut-off value of 1. This value allows for the possibility that there will still be a significant fraction of meaningful measurements in the shell, even if the majority may be below the noise threshold. This does make the tacit assumption that the unmeasured reflections are distributed randomly in reciprocal space, which is clearly not entirely true, but it's hard to see how one could account for the non-random distribution. Again, in any case I find the exact resolution cut-off makes very little difference to the maps, so it's really not worth arguing over! It's easy to show that: <I / sigma(I)>_all = shell_completeness * <I / sigma(I)>_measured So low completeness shells automatically get cut by the <I / sigma(I)> criterion. Cheers -- Ian On 6 August 2012 12:21, Marcus Fislage <[email protected]> wrote: > Dear all, > > We have in our lab a data set collected and are discussing where to cut > the resolution for refinement. According to the work of Kai Diederichs > and Andy Karplus one should use CC 1/2 of 12.5% (in case it is > significant) to determine the highest resolution independent of the > I/sigI and R factor rules used earlier. But I would like to know if this > also counts for low completeness data? > The problem is that we have in the highest resolution shell an I/sigI of > 4, a good cc1/2 but only a completeness of 30%. Which I guess means we > measured the high resolution data very accurate but not complete. Would > you still use the low complete data in the highest resolution shell or > should that be still a valid argument to cut your data towards lower > resolution? > My guess would be to use the data still even if the completeness drops, > since the data we measured is good and according to CC1/2 significant. > Are we right to do so or would you disagree? > > Thanks for any input > Marcus > > -- > Marcus Fislage > Structural Biology Brussels > Vrije Universiteit Brussel > Department of Structural Biology, VIB > Oefenplein, Gebouw E > Pleinlaan 2, > 1050 Brussel > Belgium > Tel: +32-2-629 18 51 > Email : [email protected] > Url: http://www.verseeslab.structuralbiology.be
