Impact Factors, Citations and the h-index

As impact factors, citation analyses and the 'h-index' are used increasingly to measure research impact (see last week's post on university rankings), it is worth posting a reminder of what these are and of how the measurements are made.

Journal Impact Factors
Journal impact factors provide an objective measurement for differentiating between journals by measuring the frequency with which the average article in a journal has been cited in a particular year. An impact factor of 1.5 for a journal would mean that, on average, articles published in that journal during the previous two years have been cited 1.5 times. In the University of Limerick you can find a journal's impact factor in ISI's Journal Citation Reports (see the library list of databases).

Citation Analysis
A citation analysis measures the number of times an author, article, journal or institution is cited in academic literature. In the University of Limerick a citation analysis can be done in ISI's Web of Science (see the library list of databases). Do a search for the author, giving 'Univ Limerick' as the address. At the top right of your results list click 'Create Citation Report'. The citation report will provide graphs and statistics of the author's publication and citation trend, including the average number of times an item has been cited.

h-index
Included in the Web of Science citation report, the h-index reflects both the number of publications and the number of citations per publication. A h-index of h signifies that the author has published h papers each of which has been cited at least h times. So, for example, a h-index of 4 signifies that the author has published 4 papers each of which has been cited at least 4 times. The author may in fact have published 5 or even 100 papers, but papers 5 -100 have less than 4 citations and are therefore discounted. For further information on the h-index see http://en.wikipedia.org/wiki/Hirsch_number

Rankings using the measures above are based on the premise that if an academic or institution shows good citation metrics, it is very likely that the academic or institution has made a significant impact on the academic world. However, it is important to note that the reverse is not necessarily true and that an academic or institution with weak citation metrics may, for example, be working in a small field, publishing in a language other than English, or publishing mainly in books.