Can apply to: Journal articles, preprints
Metric definition: The number of times that a journal article or preprint has appeared in the reference list of other articles and books.
Metric calculation: Many citation databases use a combination of text-mining and manual classification to build their lists of citations, based upon the reference lists of articles and books that they index. However, the scope of these databases varies, with Web of Science being the most selective (in terms of the quantity of journals and disciplines covered) and Google Scholar being the least selective (indexing a great deal of non-peer-reviewed content and various research output types). Outside of Google Scholar, it is difficult to track citations to unpublished articles (preprints).
Data sources: Popular sources of citations to journal articles include Web of Science, Scopus, and Google Scholar.
Appropriate use cases: There are many, many reasons why scholars cite each others’ work, so it’s impossible to say that there is one way that citations should be interpreted. The closest one can get is to say that citations are a measure of influence amongst other scholars, and that influence can sometimes be negative (especially in the humanities). Citations to journal articles are generally better applied to the evaluation of STEM research, given the dearth of coverage of humanities, arts, and social sciences research in most citation databases.
Limitations: One needs to read the context of a citation to understand its true meaning. Citation databases like Web of Science and Scopus have been recognized to have limited coverage of humanities, arts, and social sciences research as compared to the sciences, as well as limited coverage of local and specialized journals, especially those written in languages other than English. Moreover, differences in authorship norms between disciplines–some fields regularly have dozens of authors for a paper, where others tend to have single-author papers–mean that citations cannot always measure the full extent of an author’s contributions towards a work.
Inappropriate use cases: Citation counts should never be interpreted to directly measure quality. Raw citation counts should not be used as a measure of positive esteem for researchers.
Available metric sources: see Data sources, above
Transparency: Citations are only as transparent as the availability of the citing article or book allows them to be. One may not always be able to read citations in context, given the prevalence of subscription journals to which reviewers are not guaranteed access. Most databases that report citations report the full list of citing articles, at the very least, linking through to full-text articles where possible (even if only for subscribing institutions).
Timeframe: In theory, it is possible to track citations to journal articles as far back as the advent of the scientific journal. But in practice, most databases only track citations for articles published in the mid-1800’s and later.