Bibliometrics Definitions and Background

Citation Counts

Citation counts are the number of citations of:

  • an author's publications
  • a single document (e.g. journal article, book, chapter etc.)
  • a journal

These counts vary according to how each database does the measuring. 

Publication Output

The publication output is the number of publications produced by an author, a research unit, an institution, or a journal.

It may comprise a list of publications, or just a total number.


The h-index measures a researcher's standing by combining productivity and citation counts. It is generally seen as a measure of the researcher's entire career impact and tends to favour researchers who have published a larger number of papers. A higher h-index implies greater impact.

A researcher or journal with an h-index of x has published x articles, each of which has been cited at least x times. For example, a researcher has an h-index of 10 if 10 of their published articles have each been cited at least 10 times. The author has probably published more than 10 articles, but for the h-index to increase to 11, 11 of the author's articles need to be cited 11 or more times.

The h-index is not an average of citations per article. An author with a small number of highly-cited papers may have an artificially high number of average citations. Their h-index reflects the fact that they have published few papers.

Variations on the h-index exist. For example, the h5-index is the h-index for articles published in the last 5 complete years, which might be a fairer measure for new and emerging researchers.

The h-index is used to compare researchers' or journals' publication impact, but only within the same discipline, as publishing and citation practices differ among disciplines.