Keywords are words and phrases used to name an article’s key concepts for search and retrieval purposes. Typically an author or a publisher will assign a small number of key terms to expand lookup beyond full text or to point up the most important topics described in an article. Indexers assigning keywords can make sure that someone searching for “this topic” will find this article, even if the exact words are not present in the article’s text.
In this Tag Set, keywords come in sets (<kwd-group>), each of which may come from a particular source or ontology (such as “author-created” or the “MeSH Subject Headings”), which is named using the Keyword Authority attribute. Here are some sample tagged keywords that a contributor chose as best describing an article:
<kwd-group kwd-group-type="author-created"> <kwd>acid precipitation</kwd> <kwd>acid rainfall</kwd> <kwd>smelting region</kwd> <kwd>Aluminum residues</kwd> <kwd>Sulphur dioxide</kwd> <kwd>Copper-nickel smelters</kwd> </kwd-group>
Versions of this Tag Set prior to 3.0 have allowed for multiple sets of keywords, but the individual keywords in the set had no structure; they were just text, words, and phrases — possibly with face markup, superscripts, and subscripts (such as “<kwd>XML</kwd>”, “<kwd>H<sub>2</sub>O</kwd>”, and “<kwd>blood-brain barrier</kwd>”). Version 3.0 now accommodates more elaborate keyword structures.
Some keywords possess an internal structure of their own, for example, a keyword may include both a textual phrase and its corresponding code (“863 Icelandic sagas”). Many styles of such compound keywords can be handled in this Tag Set with the <compound-kwd> element, which is modeled as a series of repeatable parts (<compound-kwd-part>). These parts can differentiate a text/code pair, divide a coded keyword into multiple code segments, describe a hierarchy, and name a variety of other compound structures. The @content-type attribute on the <compound-kwd-part> element is used to name each part, describe the role it plays, or otherwise define how each part functions within the keyword as a whole.
The simplest case of a compound keyword is a keyword that includes both a textual phrase and its corresponding code, for example, “863 Icelandic sagas”. Both the code and the text can be tagged as keywords parts (<compound-kwd-part>) inside the element <compound-kwd>, with the @content-type attribute used to name the role or type of each part:
<kwd-group kwd-group-type="ISO-463"> <compound-kwd> <compound-kwd-part content-type="ISO-463-code">863</compound-kwd-part> <compound-kwd-part content-type="ISO-463-text">Icelandic sagas</compound-kwd-part> </compound-kwd> ... </kwd-group>
Compound keywords can also be used to handle keywords that hold an abbreviation and its expansion. Both the abbreviation and the expansion are tagged as <compound-kwd-part> in a single <compound-kwd>. The @kwd-group-type attribute on <kwd-group>, which is sometimes used to name the source or the descriptor for the keywords, can be used instead to name the type of information, such as “abbreviations”. For example:
<kwd-group kwd-group-type="abbreviations"> <compound-kwd> <compound-kwd-part content-type="abbrev">WT</compound-kwd-part> <compound-kwd-part content-type="expansion">WildType</compound-kwd-part> </compound-kwd> <compound-kwd> <compound-kwd-part content-type="abbrev">CFU</compound-kwd-part> <compound-kwd-part content-type="expansion">Colony-forming unit</compound-kwd-part> </compound-kwd> </kwd-group>
Some publishers assign hierarchical topics to articles. For example, a publisher might tag selected topics (“Blood brain barrier”), nested inside themes (“Cellular and Molecular Biology”), grouped into larger units like “Neuroscience”, and grouped into still larger units such as “Biological Sciences”, forming the following hierarchy:
Biological Sciences Neuroscience Cellular and Molecular Biology Blood–brain barrier
This kind of structure places an article in context or sorts articles into categories. This is commonly seen in Tables of Contents, where all the Neuroscience articles are grouped together and all the Biochemistry articles are grouped, etc. Since keywords are intended for searching, exact lookup, and retrieval of an article rather than establishing an article’s context, best practice, is to tag this topic structure as subject groups:
<article-categories> <subj-group subj-group-type="keywords"> <subject>Biological Sciences</subject> <subj-group subj-group-type="keywords"> <subject>Neuroscience</subject> <subj-group subj-group-type="keywords"> <subject>Cellular and Molecular Biology</subject> <subj-group subj-group-type="keywords"> <subject>Blood–brain barrier</subject> </subj-group> </subj-group> </subj-group> </subj-group> </article-categories>
If an archive wishes to tag these hierarchical structures as keywords, the following tagging would still be adequate to this Tag Set. Although <compound-kwd-part> is not recursive, the hierarchy may be preserved by setting the @kwd-group-type to “hierarchical”, “topical”, or “subject hierarchy”, and using the @content-type attribute on each part to indicate hierarchical level:
<kwd-group kwd-group-type="hierarchical"> <compound-kwd> <compound-kwd-part content-type="level1">Biological Sciences</compound-kwd-part> <compound-kwd-part content-type="level2">Neuroscience</compound-kwd-part> <compound-kwd-part content-type="level3">Cellular and Molecular Biology</compound-kwd-part> <compound-kwd-part content-type="level4">Blood–brain barrier</compound-kwd-part> </compound-kwd> </kwd-group>
Although it is not considered best practice, in some tag sets a single keyword element has been used to hold an entire list of keywords, with connecting punctuation used to mark the boundary between individual keywords rather than distinguishing individual keywords with markup. To untangle this single list into individual keywords might require publisher-specific string parsing or even human judgement, so some receiving archives have chosen not to break the list into single keywords. The element <unstructured-kwd-group> has been added as an alternative to <kwd> inside <kwd-group> to handle these lists. The element is named <unstructured-kwd-group> to indicate that it is a grouping of keywords and not a single keyword. Such lists can then be tagged as shown in the following examples:
<kwd-group> <unstructured-kwd-group>XML, DTD, schema, RELAX NG, XSD, models, UML, Schematron</unstructured-kwd-group> </kwd-group> <kwd-group> <unstructured-kwd-group>molecular chaperones; surface plasmon resonance; dynamic light scattering; trypsin digestion; citrate synthase; <italic>Neurospora crassa</italic>; protéines chaperonnes, résonance des plasmons de surface; diffusion dynamique de la lumière; digestion par la trypsine; citrate synthase; <italic>Neurospora crassa</italic></unstructured-kwd-group> </kwd-group>