International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. F. ch. 19.5, p. 448   | 1 | 2 |

Section 19.5.7.3. Data-to-parameter ratio

R. Chandrasekarana* and G. Stubbsb

aWhistler Center for Carbohydrate Research, Purdue University, West Lafayette, IN 47907, USA, and  bDepartment of Molecular Biology, Vanderbilt University, Nashville, TN 37235, USA
Correspondence e-mail:  [email protected]

19.5.7.3. Data-to-parameter ratio

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The total number of data used in this refinement process is M + I + J, where M, I and J are, respectively, the number of observations in the first three terms of equation (19.5.7.1[link]). If P is the number of parameters refined and H is the number of independent constraints in the last term, then the number of degrees of freedom of the system is [P - H]. The effective number of data is given by [D = (M + I + J) - (P - H)]. The data-to-parameter ratio (D/P), a measure of the dependability of the final results, must be greater than one for meaningful refinement. D/P is typically in the range 3 to 11 in the analysis of polynucleotide and polysaccharide structures. This ratio is comparable to those commonly reported for single-crystal structures, confirming that fibre-diffraction analysis of polymers, despite the limited number of X-ray data, can yield reliable results.








































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