For scientists who fear being reduced to a statistic, a theoretical physicist has some unsettling news: A single, simply calculated number provides a rough estimate of the impact of any particular scientist's work, he says.
From deciding whether to grant tenure to a junior faculty member to selecting the winner of a prestigious prize, researchers often face the challenge of objectively evaluating the importance of a colleague's work. Committees may weigh quantitative measures of a scientist's productivity, such as the number of papers published or the number of citations those papers have amassed. But interpreting these numbers can be tricky. For example, a mediocre scientist might crank out numerous papers that few people read. Conversely, a researcher might co-author just a few articles that are cited hundreds of times, inflating his or her number of citations. But one easily determined number provides a more reliable measure of the impact of a researcher's work, says Jorge Hirsch of the University of California, San Diego.
In a paper posted to the preprint server www.arxiv.org, Hirsch defines a researcher's index, h, as the largest number such that the researcher has h papers with at least h citations each. For example, string theorist Edward Witten of the Institute for Advanced Study in Princeton, New Jersey, has 110 papers with 110 or more citations each, making his h index 110. The h index favors researchers who consistently produce influential papers and disfavors those who publish many little-noted papers or just a few widely cited ones, says Hirsch (h = 49): "I can't imagine a person with a high h index who hasn't done important work."
"The great advantage of this h index is the fact that you can get it in about 30 seconds" using the ISI Web of Knowledge, an online citation index, says Manuel Cardona (h = 86), a physicist at the Max Planck Institute for Solid State Research in Stuttgart, Germany. Sidney Redner (h = 39), a physicist at Boston University, agrees that the index is "clever" but notes that the average h index may vary widely from field to field. And, he says, "I'm not sure I'd want to base people's tenure decisions on whether their h index is above a particular level."