What do we know about the h index?

recently proposed the h index to quantify the research output of individual scientists. The new index has attracted a lot of attention in the scientific community. The claim that the h index in a single number provides a good representation of the scientific lifetime achievement of a scientist as well as the (supposed) simple calculation of the h index using common literature databases lead to the danger of improper use of the index. We describe the advantages and disadvantages of the h index and summarize the studies on the convergent validity of this index. We also introduce corrections and complements as well as single-number alternatives to the h index. In evaluative bibliometrics, the measurement of research performance at the micro level, that is, at the level of an individual scientists, is viewed as problematic (Cole, 1989). The reasons are that (a) a sufficiently large publication output produced in a manageable time span is necessary to obtain statistically reliable indicators, and (b) research productivity, publication numbers, and citation impact are not necessarily correlated variables (Glänzel, 2006b). However, due to scarce resources the quantification of scientific performance is needed for evaluation and comparison purposes to inform funding or tenure decisions (Ball, 2005; Hirsch, 2005a). Jorge Hirsch (2005a, 2005b) recently proposed a new research performance indicator that is designed for application at the micro level. The Hirsch index, or h index, quantifies as a single-number criterion the scientific output of a single researcher. Hirsch's index is an original and simple new measure incorporating both quantity and visibility of publications van Raan, 2006): " A scientist has index h if h of his or her N p papers have at least h citations each and the other (N p Ϫ h) papers have fewer than Յ h citations each " (Hirsch, 2005a, p. 16569; see also Rousseau, 2006b, 2006c). A h index of 40 means, for example, that a scientist has published 40 papers that each had at least 40 citations. A scientist's h index will never decrease (Sidiropoulos, Katsaros, & Manolopoulos, 2006); an increase is to be expected as new (high-impact) papers are published, as " sleeping beauties " (van Raan, 2004b) come to life, and as the scientist's papers attract citations (Cronin & Meho, 2006; Hirsch, 2005a). h ϭ 0 characterizes inactive scientific authors (Glänzel, 2006b) that have at best published papers that have had no visible impact. The proposed new measure …