Elsevier Library Connect Newsletter: Jorge Hirsch on Overall, I’m more interested in physics than citations

Volume 5 • Number 3
August 2007 • ISSN: 1549-3725
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Overall, I’m more interested in physics than citations
By Jorge Hirsch, Physics Professor, University of California San Diego, USA

Jorge Hirsch (on the left) confers with
Elsevier Senior Product Technology
Manager M'hamed el Aisati.

Many people ask me why I came up with the “highly cited index” or h-index, a method for quantifying a scientist's publication productivity and impact. Basically, the truth is that I dislike impact factors because, due to the controversial nature of my articles and research, I’m unable to get my work published in journals with high impact factors. Despite this, many of my articles have received large numbers of citations.

Background of the h-index

At many institutions, including my own, citation counts are considered during decisions relating to hiring, promotion and tenure. Despite the fact that citation counts can contain misinformation, for example, when many co-authors or self-citations are involved, they form a basic quantitative measure of a researcher’s output and impact. Hence citation counts should play an important role in evaluations, even if (or maybe especially when) the papers are not published in “high-impact journals.”

The h-index is about providing a simple objective measure for research evaluation.

In the summer of 2003, I first discussed the concept and mathematical calculation of the x-coefficient, as I initially called the h-index, with some colleagues at UCSD, and started to use it informally in evaluations. I wrote up a draft paper but wasn't sure it would be of sufficient interest for publication. In the spring of 2005, I sent the paper to some colleagues and asked for comments. Some time later, a colleague from Germany emailed me inquiring about the index and expressing great interest. Then I decided to upload my h-index paper¹ onto the Los Alamos server, which I did on August 3, 2005. I was still not sure whether to publish it in a refereed journal. To my surprise, the preprint received a very high level of interest. Before long, I found my email box filled with comments related to the article.

In essence, the h-index is about providing a simple objective measure for research evaluation. Since it is not related to the popularity of a journal, this index is a way to put more democracy into research performance measurements. In fact, papers that receive high numbers of citations in “low-impact” journals should be especially noteworthy.

Possible improvements to the h-index

Naturally no single quantitative measurement is sufficient on its own. One can add other features of the citation distribution besides the h-index to reflect additional citation information. For example, one may also consider the slope (first derivative) and curvature (second derivative) of the distribution, as well as the integral (total number of citations), as additional criteria. In the relation N_Total = ah², a is normally between 3-5, but deviations do occur.

The h-index does not normalize for the number of years that a researcher has been active. This can be done by dividing by the time since graduation or receipt of a PhD: h(t) = mt (where m is expected to be approximately time independent). It is also interesting to normalize the h-index taking into account the number of co-authors. Furthermore there are variations in the h-index between different disciplines and subdisciplines.

Will I continue my investigation into indicators of research evaluation? To some extent, yes; however overall, I am more interested in physics than citations.

¹Hirsch, J. E. (2005, September 29). An index to quantify an individual’s scientific research output.
http://arxiv.org/abs/physics/0508025

Scopus harnesses the h-index

You may have already noticed that the h-index has been incorporated into Scopus. The index considers the publication records of an individual, the number of papers published over n years and the number of citations for each paper, and produces a single number, the h-index. To assist users interested in this index, Scopus offers a set of visual aids (see example below) presenting a transparent overview of citation and publication patterns over time and revealing whether an author’s h-index is dependent on a few highly cited papers or if an author’s papers have drawn a relatively consistent volume of citations.

http://labs.elsevier.com/scopusinfo/search.jsp?q=h-index