What the h ?
I
magine if a research career could be summed up in a single number. And imagine further that the number was freely and publicly available. Be careful what you wish for because . . . voilà, it may appear. Actually, it did appear on November 15, 2005, in the Proceedings of the National Academy of Sciences U.S.A. (102, 16,569–16,572) in a paper by physicist Jorge Hirsch from the University of California San Diego titled “An Index To Quantify an Individual’s Scientific Research Output”. Hirsch invented the h-index, and the world of evaluating scientific contributions has never been the same. In case you’ve been living under a rock these past 2 years, the h-index is the number of papers (h) from an individual’s total publications Np that have been cited at least h times. Hirsch determined that Nobel Prize winners in physics have an average h-index of 41, that is, 41 of their papers were cited at least 41 times. The highest h-index Hirsch could find among scholars in his field went to E. Witten, h = 110, whereas h = 62 characterized the famous physicist Stephen Hawking. Of course, there are differences among fields. Disciplines develop different standard practices and cultures; those fields that cite more references in a typical paper or that have more authors per paper will produce greater h-indices. In particular, life scientists generate large h-indices, and the highest h-index for any scientist that Hirsch could find was 191 for S. Snyder, a biologist. It’s relatively easy to estimate the h-index for a friend or colleague (or a competitor). From the ISI Web of Knowledge homepage, simply rank the author’s papers by “times cited” and scroll down to the paper with the same number of citations. Yours truly is not above a vanity search, so I can tell you I’ve accumulated a healthy h-index of 26; that is, 26 of my papers have been cited at least 26 times. But just when I was feeling pretty good, I looked up ES&T Associate Editor Ron Hites. Imagine my chagrin when I discovered that Ron has an h-index of 60. Zounds! Is Ron twice the man that I am? All of this threw me into a major funk. But I came to realize that we at ES&T have, indeed, a vibrant culture of scholarly publication that fits somewhere between the typical pure sciences and the applied engineering fields. Engineers cite fewer references per paper and have a lower average h-index than do pure scientists. Women, who contribute immensely to our jour-
© 2008 American Chemical Society
nal, tend to be younger than the male contributors, and their h-indices are rising fast, for example Lynn Hildemann’s (h = 26). The average h-index for corresponding authors in ES&T in one randomly selected issue (January 1, 2008) is 15, a very respectable number. That’s one reason why ES&T enjoys a high impact factor among journals in its field. Still, I find something rather distasteful about reducing an entire scientific career down to a single number. My biggest concern about h is that it’s so compact and convenient that it can be used in idiotic ways. The h-indices come in handy for department heads and deans who know the h of everyone but the value of no one. I have heard of administrators who calculate the h-index for entire departments or even universities, then use it blindly to give salary increments, allocate resources, or furlough whole departments. What the h? The h-index is a useful data point, but it’s only one piece of information in a complex evaluation of research impacts. In the meantime, physicist Hirsch has spawned a whole new industry of researchers and consultants trying to improve on his index. Now we have Egghe’s g-index, the Contemporary h-index, the ageweighted citation rate (AWCR), and two variations of the Individual h-index to normalize for coauthorships. So how should I respond if my department administrator says my h-factor is too small? I would counter that my h-factor is just fine; it’s really my salary that is too small. Shouldn’t the measure of a person’s career include both the inputs (salary, quality of facilities and students) as well as the outputs (citations, awards, patents, designs)? To the rescue came my colleague Rich Valentine. He invented a new inverse number, the phd-index, defined as the negative logarithm of one’s h-index divided by salary in U.S. dollars. The lower the number, the larger the value-to-price ratio. It’s a wonderful creation; at least he thinks so. His index is (now) better than mine.
Jerald L. Schnoor Editor
[email protected] May 1, 2008 / Environmental Science & Technology ■ 3121
