3.1. General considerations when defining a CIF data item

Abstract

CIF dictionaries provide a formal taxonomy of crystallographic terms and ideas. Dictionary entries are constructed in a structured machine-readable manner that facilitates validation and structuring of data. New entries may be devised for public or private dictionaries. A candidate data-name definition should fulfil the following conditions:

• It should describe a specific and well defined concept: precision of definition is essential for an effective interchange mechanism.
• It should have appropriate granularity: data names can define a very small piece of information (e.g. a standard uncertainty on a particular physical measurable) or a very large amount of information (e.g. the text of a scientific paper). An appropriate choice should be made (and for DDL2 the choice should be formalized through membership of subcategories, category and category groups, as appropriate).
• It should have well defined relationships with other data items (through its assigned category membership and parent/child links); for DDL2 the prior construction of a formal entity/relationship schema may be helpful.
• Constraints on the data type and permissible values should be provided where applicable.
• The name chosen should be globally unique; this is achieved through monitoring of names in public dictionaries by a regulatory committee (COMCIFS) and by registering of prefix strings for exclusive use in local dictionaries.

Some thought may need to be applied to the choice of DDL appropriate for a candidate dictionary. Rules are described for the structuring of dictionaries and the protocol for merging separate dictionary files to provide access to a global distributed dictionary is outlined.