# Changes

,  07:07, 18 June 2008
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The following is based on a CCP4BB discussion around June 17, 2008 entitled: "How many reflections for Rfree?"
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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>]]"
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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.
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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.
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==How precise is the estimate of [[iucr:Free_R_factor|R<sub>free</sub>]] for a certain number of test set reflections?==
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==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%.
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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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