Free R: Difference between revisions
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Free R is a statistical quantity introduced in 1992 by Axel T. Brünger<ref>Brünger AT. Free R value: a novel statistical quantity for assessing the accuracy of crystal structures. Nature. 355:472-5. [http://www.ncbi.nlm.nih.gov/pubmed/18481394 PubMed]</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 a statistical quantity introduced in 1992 by Axel T. Brünger<ref>Brünger AT. Free R value: a novel statistical quantity for assessing the accuracy of crystal structures. Nature. 355:472-5. [http://www.ncbi.nlm.nih.gov/pubmed/18481394 PubMed]</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. | ||
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. | ||