Free R: Difference between revisions
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[[Free R]] (also called R<sub>free</sub>) "is generally considered the most useful global measure of model-to-data agreement"<ref name="validation4pdb">PMID: 22000512</ref>. It is a statistical quantity introduced in 1992 by Axel T. Brünger<ref>PMID: 18481394</ref> to assess the quality of a model from X-ray crystallographic data. It is calculated in the same manner as the [[R value]], but from a subset of the data set aside for the calculation of free R, and not used in the refinement of the model. It is a more reliable tool for assessing the model than the R value because it is not self-referential -- that is, as an estimation of errors, free R is free of any bias that may have been introduced during refinement. Free R is an example of the application of the more general method of [http://en.wikipedia.org/wiki/Cross-validation_(statistics) cross validation]. | [[Free R]] (also called R<sub>free</sub>) "is generally considered the most useful global measure of model-to-data agreement"<ref name="validation4pdb">PMID: 22000512</ref>. It is a statistical quantity introduced in 1992 by Axel T. Brünger<ref>PMID: 18481394</ref> to assess the quality of a model from X-ray crystallographic data. It is calculated in the same manner as the [[R value]], but from a subset of the data set aside for the calculation of free R, and not used in the refinement of the model. It is a more reliable tool for assessing the model than the R value because it is not self-referential -- that is, as an estimation of errors, free R is free of any bias that may have been introduced during refinement. Free R is an example of the application of the more general method of [http://en.wikipedia.org/wiki/Cross-validation_(statistics) cross validation]. | ||
==Good Values for Free R== | |||
As a rule of thumb, free R should not exceed the R value by more than 0.05; that is, if the R value is 0.20, free R should not significantly exceed 0.25. Free R values exceeding 0.40 raise serious doubts about the model. | As a rule of thumb, free R should not exceed the R value by more than 0.05; that is, if the R value is 0.20, free R should not significantly exceed 0.25. Free R values exceeding 0.40 raise serious doubts about the model. | ||
Free R is positively correlated with [[resolution]]<ref name="validation4pdb" />: | Free R is positively correlated with [[resolution]]<ref name="validation4pdb" />: | ||
{| class="wikitable" | |||
|- | |||
! Resolution, Å | |||
! Median Free R<ref name="validation4pdb" /> | |||
! Free R - (Resolution/10) | |||
! Free R Best 25% | |||
! Free R Worst 25% | |||
|- | |||
| 1.0 | |||
| 0.15 | |||
| 0.05 | |||
| <0.14 | |||
| >0.17 | |||
|} | |||
* At resolution 1.0 Å, the median free R is about 0.15 ((R/10) + 0.05). | * At resolution 1.0 Å, the median free R is about 0.15 ((R/10) + 0.05). | ||
