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−

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<sub>free</sub>]]"

−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<sub>free</sub>]] 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:

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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<sub>free</sub>~~]] ~~for a certain number of test set reflections?==

+==How precise is the estimate of R<sub>free</sub> 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<sub>free</sub>]] 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<sub>free</sub>]] 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).

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