Scientists, Know Thy Hirsch Number!
The Hirsch number (otherwise known as the h-index) tries to quantify the (scientific) productivity of a scientist. The Hirsch number was first described by the physicist J.E. Hirsch in a 2005 PNAS paper: An index to quantify an individual?s scientific research output. Hirsch says:
I propose the index h, defined as the number of papers with citation number
h, as a useful index to characterize the scientific output of a researcher.The publication record of an individual and the citation record clearly are data that contain useful information. That information includes the number (Np) of papers published over n years, the number of citations (Njc) for each paper (j), the journals where the papers were published, their impact parameter, etc. This large amount of information will be evaluated with different criteria by different people. Here, I would like to propose a single number, the “h index,” as a particularly simple and useful way to characterize the scientific output of a researcher.A scientist has index h if h of his or her Np papers have at least h citations each and the other (Np ? h) papers have
h citations each.
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| Histogram giving number of Nobel-prize recipients in Physics in the last 20 years versus their h-index. The peak is at h-index between 35 and 39 (image and text: http://physicsworld.com/cws/article/news/22890/1/050809.) |
For example, if I have a Hirsch number of 10, that simply means that I have written 10 papers with at least 10 citations each.
Most of us are familiar with the impact factor, which is the commonly accepted method for measuring the scientific “weight” of a paper. The impact factor for a journal is “calculated based on a three-year period, and can be considered to be the average number of times published papers are cited up to two calendar years after publication (including the calendar year in which it was published)” (Source: Wikipedia). Currently the top scientific journal with the highest impact factor is Science (30.028) followed closely by Nature (26.681).
J.E. Hirsch further argues in a recent paper that the h-index appears to be a good predictor of one’s scientific productivity in the future.
For those of us who hate quantifying our work, and having ourselves “measured” in terms of simple numbers, it may very well be that the h-index is just another mathematical exercise that we can do without. The thought of having our CVs - our life’s work - reduced to a single number could be unnerving especially to those who are only about to embark on their scientific careers. On the other hand, knowing in advance that such a parameter exists - which could very well be the basis for evaluation in future employment applications - may also help one in mapping out the best strategies for scientific productivity and career advancement.