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Many bibliographic databases describe the content of a publication using a thesaurus. The vocabularies vary and the extent to which the databases apply them may also differ significantly. The aim of this study is to empirically explore the number of subject headings assigned to publications in two databases over time and to determine if publication characteristics are associated with the number of subject headings. Articles and reviews in MEDLINE and Embase from 1990 to 2019 assigned with one of the subject headings from six subject areas are included in this study. Each of the retrieved publications in Embase is matched with a similar publication in MEDLINE. Furthermore, multivariable linear regressions are used to explore the association of the number of subject headings in MEDLINE and Embase with six prespecified publication characteristics. The average number of assigned subject headings in MEDLINE is stable or even slightly decreasing over time. In Embase, the average number of assigned subject headings was stable until about 2000 where the average number increased dramatically during the next 3 years. Furthermore, linear regressions show that the average number of subject headings in MEDLINE and Embase is higher for publications in English, publications with longer abstract, recent publications and if it belongs to specific subject areas. However, reviews are assigned with more subject headings in Embase and fewer in MEDLINE. The implications of the results are discussed.
Frandsen et al. (Thu,) studied this question.