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In crystallography, the R-factor is a measure of the agreement between the crystallographic model and the experimental X-ray diffraction data. In other words, it is a measure of how well the refined structure predicts the observed data. It is defined by the following equation: where F is the so-called structure factor and the sum extends over all the reflections measured and their calculated counterparts respectively. The structure factor F is closely related to the intensity of the reflection it describes: For large molecules, R-factor usually ranges between 0.6 and 0.2. Small molecules usually form more ordered crystals than large molecules, it is possible to attain lower R-factors. In the Cambridge Structural Database more than 95% of the 500,000+ crystals have an R-factor lower than 0.15 and 9.5% have an R-factor lower than 0.03. Crystallographers also use the Free R-Factor to describe the quality of a model. The quantities and are similarly used to describe the internal agreement of measurements in a crystallographic data set.
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