h-index

The h-index (also known as Hirsch index) was introduced by J. Hirsch in 2005 and can be defined as follows: A researcher has an h-index, if he/she has at least h publications for which he/she has received at least h citations. For example, Researcher A has an h-index = 13 if he/she has published at least 13 documents for which he/she has received at least 13 citations. Its popularity as a bibliometric indicator has derived from the fact that it combines productivity (number of documents) and impact (number of citations) in one index.

The h-index can be applied to any level of aggregation (author, institution, journal, etc.) and it can reveal information about how the citations are distributed over a set of documents. At the author level, it is considered to be an indicator of a researcher’s lifetime scientific achievements. Some clear advantages of the h-index are that it is a mathematically simple index, it encourages large amounts of impactful research work while at the same time discourages publishing unimportant output and that single highly cited publications do not influence the h-index (unlike the Citation Impact).

However, the h-index is a time-dependent measure, as it is proportional to the length of a researcher’s career and how many articles they have published. For example, early career researchers would be at a disadvantage when compared to more senior researchers because the latter would have had more time to produce more work and receive more citations for their output.

Example of h-index at the Author Level

Total Publications

Total Citations

Citation Impact

H-Index

Researcher A

1

50

50

1

Researcher B

10

200

20

10

Researcher C

10

200

20

5

This table  shows an example of how h-index can be applied at the author level. Researcher A has only one publication that has received 50 citations, while Researcher B has published 10 documents that have received 20 citations each. Researcher C has the same number of publications and citations as Researcher B. According to the definition of the h-index, Researcher A, who has only one publication and has received 50 citations will have an h-index = 1, whereas Researcher B who has 10 publications and has received 20 citations for each publication will have an h-index = 10. Researcher C has an h-index = 5, which means that even though he/she has published the same number of documents and received the same number of citations as Research B, Researcher’s C citations are more concentrated in five documents that are more cited than the rest of his/her publications.

Note, however, that in this example we have not taken into account the researchers’ ages (the time interval between when the first and last document were published) and the disciplines that the researchers are active in. The h-index can be very different across disciplines due to the differences in the average citation rates and therefore, sensitive to the disciplinary background of research output, as research entities publish in different subject mixes.

Assessing the productivity of a large set of publications is the first step in a series of bibliometric analyses that we can apply in order to obtain a deeper understanding of the performance of our research output.