But at some point, getting a "clear map" might not be the goal. If you're in refinement mode, the weak reflections also provide information that your model needs to fit. I find <I/sig(I)> (or <I>/<sig(I)>) to be about as useful as
Rmerge (or its relatives). Ron
On Wed, 9 Jun 2010, James Holton wrote:
Frank von Delft wrote:
On 09/06/2010 16:49, James Holton wrote:
Operationally, I recommend treating anisotropic data just like isotropic
data. There is nothing wrong with measuring a lot of zeros (think about
systematic absences), other than making irrelevant statistics like Rmerge
higher. One need only glance at the formula for any R factor to see that
it is undefined when the "true" F is zero. Unfortunately, there are still
a lot of reviewers out there who were trained that "the Rmerge in the
outermost resolution bin must be 20%", and so some very sophisticated
ellipsoidal cut-off programs have been written to try and meet this
criterion without throwing away good data. I am actually not sure where
this idea came from, but I challenge anyone to come up with a sound
statistical basis for it. Better to use I/sigma(I) as a guide, as it
really does tell you how much "information vs noise" you have at a given
resolution.
So, if my outer shell has
10% reflections I/sigI>10,
90% reflections I/sigI=1,
will Mean(I/sigI) for that shell tend to 10 or 1?
Presumably I'm calculating it wrong in my simulation (very naive: took
average of all individual I/sigI), because for me it tends to 1.
But if I did get it right, then how does Mean(I/sigI) tell me that 10% of my
observations have good signal?
It doesn't. The mean will not tell you anything about the distribution of
I/sigI values, it will just tell you the average. If I may simplify your
example case to: one good observation (I/sigI = 10) and 9 weak observations
(I/sigI = 1), then Mean(I/sigI) = ~2. This is better than Mean(I/sigI) = 1,
but admittedly still not great. I know it is tempting to say: "but wait! I've
got one really good reflection at that resolution! Doesn't that "count" for
something?" Well, it does (a little), but one good reflection does not a clear
map make.
-James Holton
MAD Scientist
