I'm not sure that "a lttle worse" is the appropriate description - I think all you can say is that it deviates from the average when only resolution is used to define what is "average". My point is that a number of other factors are known to be involved and so you can't say that this deviation is "worse" until you have taken them all into account.
Specifically, it's known that the expected value of the ratio Rfree/Rwork (i.e. expected on the basis of the null hypothesis that the structure is correct and complete and the only errors are random experimental errors) is directly related to the ratio (no of significant experimental observations) / (effective no of refined parameters), where "effective" here means "taking into account the restraints". The number of significant observations will clearly depend on a number of factors, not only resolution, but also solvent content (which you obviously can't control unless you use a different crystal form), and data completeness (which you can control up to a point by optimising the data collection strategy). The effective no of parameters obviously depends only on the parameter/restraint model and the weights, both of which you have full control of, and therefore the effective no of parameters should be completely determined if the parameter/restraint model that has been selected is optimal. Finally, in order to compute Rfree/Rwork from Rdiff = Rfree-Rwork, Rwork itself must be specified, i.e.: Rfree/Rwork = (Rfree-Rwork) / Rwork - 1 = Rdiff/Rwork - 1 It's actually much easier to work with Rfree/Rwork instead of Rdiff, because then you don't need to specify a particular value of Rwork, and you have one less variable to worry about in the factor analysis. So assuming the model is optimal, the major factors in addition to resolution which control the expected value of Rdiff are the solvent content, the data completeness and Rwork. The value of Rwork obtained for an optimal model on convergence is obviously related to the data quality (e.g. mean I/sig(I)), and of course the resolution. The bottom line is that unless we are given a lot more information it's not possible to say whether a specific value of Rdiff deviates significantly from the expected value. Cheers -- Ian > > Then for all structures a we see that Rdiff of > 0.7 is not that uncommon > with this about 1 sigma away from the mean value of 0.4 for all structutures > and 0.45 for your resolution range > > For structures with Rdiff range of 0.7-0.8 and resolution 2.7 - 2.9 we see > that there 212 structures. > > If I edit the query in PDBeDatabase to your exact requirement ranges then > there > are 1353 example structures out of 53616 examples where this data exists. > > My comment is that this is little worse than average, but not particularly a > problem. > > Tom >> >> Hi all, >> >> Can anyone comment, in general, on diverging Rcryst and Rfree >> values(say>7%) for structures with kind of low resolutions(2.5-2.9 >> angstroms)? >> >> Thanks >> RJ >> >
