International
Tables for
Crystallography
Volume G
Definition and exchange of crystallographic data
Edited by S. R. Hall and B. McMahon

International Tables for Crystallography (2006). Vol. G, ch. 3.1, pp. 83-84

Section 3.1.7.1. Granularity

B. McMahona*

aInternational Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England
Correspondence e-mail: bm@iucr.org

3.1.7.1. Granularity

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Perhaps the most obvious decision that needs to be made is the level of detail or granularity chosen to describe the topic of interest. CIF data items may be very specific (the deadtime in microseconds of the detector used to measure diffraction intensities in an experiment) or very general (the text of a scientific paper). In general, a data name should correspond to a single well defined quantity or concept within the area of interest of a particular application. It can be seen that the level of granularity is determined by the requirements of the end application.

A practical example of determining an appropriate level of granularity is given by the core dictionary definitions for bibliographic references cited in a CIF. The dictionary originally contained a single character field, _publ_section_references, which was intended to contain the complete reference list for an article as undifferentiated text. Notes for Authors in journals accepting articles in CIF format advised authors to separate the references within the field with blank lines, but otherwise no structure was imposed upon the field. In a subsequent revision to the core dictionary, the much richer CITATION category was introduced to allow the structured presentation of references to journal articles and chapters of books. This was intended to aid queries to bibliographic databases. However, a full structured markup of references with multiple authors or editors in CIF requires additional categories, so that the details of the reference may be spread across three tables corresponding to the CITATION, CITATION_AUTHOR and CITATION_EDITOR categories. Populating several disjoint tables greatly complicates the author's task of writing a reference list. Moreover, the CITATION category does not yet cover all the many different types of bibliographic reference that it is possible to specify, and is therefore suitable only for references to journal articles and chapters of books. However, it is possible to write a program that can deduce the structure of a standard reference within an undifferentiated reference list (provided the journal guidelines have been followed by the author) to the extent that enough information can be extracted to add hyperlinks to references using a cross-publisher reference linking service such as CrossRef (CrossRef, 2004[link]). Therefore, in practice, IUCr journals still ask the author of an article to supply their reference list in the _publ_section_references field, rather than using the apparently more useful _citation_ fields. It remains to be seen whether this is the best strategy in the long term.

In more technical topic areas, the details of an experimental instrument could be described by a huge number of possible data names, ranging from the manufacturer's serial number to the colour of the instrument casing. However, many of these details are irrelevant to the analysis of the data generated by the instrument, so the characteristics of an instrument that are assigned individual data names are typically just those parameters that need to be entered in equations describing the calibration or interpretation of the data it generates.








































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