The h-index was proposed by Hirsch (2005). It is defined as follows:
A scientist has index h if h of his/her Np papers have at least h citations each, and the other (Np-h) papers have no more than h citations each.
It aims to measure the cumulative impact of a researchers output by looking at the amount of citations his/her work has received. The properties of the h-index have been analyzed in various papers; see for example Egghe & Rousseau (2006).
Since Hirsch's first publication of the h-index in 2005, this new measure of academic impact has generated a very widespread interest. At the time of writing (July 2010) Google Scholar lists 1180 citations to this paper and the subsequent publication in the Proceedings of the National Academy of Sciences.
Hirsch argues that the h-index is preferable to other single-number criteria, such as the total number of papers, the total number of citations and citations per paper. However, Hirsch provides a strong caveat:
Obviously a single number can never give more than a rough approximation to an individuals multifaceted profile, and many other factors should be considered in combination in evaluating an individual. This and the fact that there can always be exceptions to rules should be kept in mind especially in life-changing decision such as the granting or denying of tenure.
The advantage of the h-index is that it combines an assessment of both quantity (number of papers) and quality (impact, or citations to these papers) (Glänzel, 2006). An academic cannot have a high h-index without publishing a substantial number of papers. However, these papers need to be cited by other academics in order to count for the h-index.
As such the h-index is said to be preferable over the total number of citations as it corrects for “one hit wonders”, i.e. academics who might have authored (or co-authored, maybe even as 5th or later author) one or a limited number of highly-cited papers, but have not shown a sustained and durable academic performance. It is also preferable over the number of papers as it corrects for papers that are not cited. Hence the h-index favours academics that publish a continuous stream of papers with lasting and above-average impact. (Bornmann & Daniel, 2007).
Validation and reception of the h-index
The h-index has been found to have considerable face validity. Hirsch calculated the h-index of Nobel prize winners and found 84% of them to have an h-index of at least 30. Newly elected members in the National Academy of Sciences in Physics and Astronomy in 2005 had a median h-index of 46.
Bornmann & Daniel (2005) found that on average the h-index for successful applications for postdoctoral research fellowships was consistently higher than for non-successful applicants. Cronin & Meho (2006) found that faculty rankings in information sciences based on raw citation counts and on the h-index showed a strong positive correlation, but claim that the h-index provides additional discriminatory power.
Van Raan (2006) calculated the h-index for 147 chemistry research groups in the Netherlands and found a correlation of 0.89 between the h-index and the total number of citations. Both the h-index and more traditional bibliometric indices also related in a quite comparable way with peer judgments.
The h-index has resulted in a flurry of commentaries and articles published in journals such as Scientometrics and Journal of the American Society for Information Science and Technology, including several articles proposing further refinements and alternatives (see the sections on the contemporary an individual h-index below) and in spite of strong criticism by some bibliometricians, it has generally received a positive reception.
Perhaps the strongest indication that the h-index is becoming a generally accepted measure of academic achievement is that ISI Thomson has now included it as part of its new citation report feature in the Web of Science.
Disadvantages of the h-index
A disadvantage of the h-index is that it ignores the number of citations to each individual article over and above what is needed to achieve a certain h-index. Once a paper belongs to the top h papers, its subsequent citations no longer count. Hence, in order to give more weight to highly-cited articles Leo Egghe (2006) proposed the g-index (see Section 1.4.6 below for more details).
The h-index (and any citation-based measures) is a less appropriate measure of academic achievement for junior academics, as their papers have not yet had the time to accumulate citations. Especially in the Social Sciences & Humanities it might take five to ten years before a paper acquires a significant number of citations. For junior academics, the impact factor of the journal they publish in might therefore be a more realistic measure of eventual impact.
However, the h-index should provide a more realistic assessment of the academic achievement of academics that have started publishing at least 10 years ago. I would argue that for more senior academics, assessing the impact of their own publications is preferable to assessing the journal impact factor of the journals they publish in. The latter is only a measure of how of the average article in the journal is cited.