View source for R value

The R value is used to assess progress in the refinement of a model from X-ray crystallographic data, and can be used as one factor in [[Quality assessment for molecular models|evaluating the quality of a model]]. R is a measure of error between the observed intensities from the diffraction pattern and the predicted intensities that are calculated from the model. R values of 0.20 or less are taken as evidence that the model is reliable.

As a rule of thumb, models with R values substantially exceeding (resolution/10) should be treated with caution. Thus, if the resolution of a model is 2.5 Å, that model's R value should not exceed 0.25. Completely erroneous models (e.g. random models) give R values of 0.40 to 0.60.

However, R values themselves must be treated with caution. Unlike the [[Free R]], acceptable R values can be achieved despite serious errors in the model, as demonstrated unequivocally by Kleywegt & Brünger<ref>PMID: 8805582</ref>. In fact analysis has shown that this value is frequently underestimated so that a final model is not as good as it should be<ref name="Karplus">PMID: 22628654</ref><ref>PMID: 22628641</ref>.  A different statistical value, CC*, has been proposed to  assess model and data quality on the same scale and reveal when data quality is limiting model improvement <ref name="Karplus" />.

One famous pitfall that can result in a misleading R value is the addition of substantially more than one water molecule per amino acid.

==See Also==

*[[Free R]]
*[[Resolution]]
*[[Temperature value]]
*[[CC*]]
*[[Quality assessment for molecular models]]

==Content Donors==

Portions of this page were adapted from the [http://proteinexplorer.org/igloss.htm Glossary of ProteinExplorer.Org], with the permission of the principal author, [[User:Eric Martz|Eric Martz]].

==Literature==

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