Changes

The following is based on a CCP4BB discussion around June 17, 2008 entitled: "How many reflections for Rfree?"

The following is based on a CCP4BB discussion around June 17, 2008 entitled: "How many reflections for [[iucr:Free_R_factor|R_free]]"

First of all, the test set is that set of reflections put aside for unbiased calculation of statistical quantities, in particular R_free and sigmaA.

First of all, the test set is that set of reflections put aside for unbiased calculation of statistical quantities, in particular [[iucr:Free_R_factor|R_free]] and sigmaA.

The need to find a good compromise for the size of the test set has been discussed by Axel Brunger in a "Methods in Enzymology" (1997) paper. He writes:

The need to find a good compromise for the size of the test set has been discussed by Axel Brunger in a "Methods in Enzymology" (1997) paper. He writes:

  and the need to avoid a deleterious effect on the atomic model by omission of too much experimental data.

  and the need to avoid a deleterious effect on the atomic model by omission of too much experimental data.

==How precise is the estimate of [[iucr:Free_R_factor|R_free]] for a certain number of test set reflections?==

==How precise is the estimate of R_free for a certain number of test set reflections?==

The estimate for the relative error of R_free is 1/sqrt(n), where n is the size of the test set. So if n is 1000, and the R_free is 31%, you would expect its relative error to be 31%/sqrt(1000), which is about 1%.

The estimate for the relative error of [[iucr:Free_R_factor|R_free]] is 1/sqrt(n), where n is the size of the test set. So if n is 1000, and the [[iucr:Free_R_factor|R_free]] is 31%, you would expect its relative error to be 31%/sqrt(1000), which is about 1%.

I believe this is from a paper of Ian Tickle (FIXME: reference).

I believe this is from a paper of Ian Tickle (FIXME: reference).