Re: [phenixbb] Optimizing the Geometry Weight for a 2.76 A structure

Phil Jeffrey wrote:

More pertinent to this example is the s.d. of the R-free itself, i.e.

sigma(Rfree)/Rfree ~ 1/sqrt(Ntest)

I believe "~" here is read "proportional to", not "approximately". Ed PS- Rfree can be seen* as an average of (|Fo-Fc|)/Fo weighted by Fo, so variance should be proportional to sqr(N) like the standard error of the mean. But SEM depends also on the sigma for the individual observations, which I would hope is less than 100%, so sigma(Rfree)/Rfree < 1/sqrt(Ntest)? *derivation: unweighted average: Sum(|Fo-Fc|/Fo)/N = Sum(|Fo-Fc|/Fo)/Sum(1) weighted by Fo: Sum(Fo*|Fo-Fc|/Fo)/Sum(Fo) = Sum(|Fo-Fc|)/Sum(Fo) = R