h-index

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The h-index is an index that attempts to measure both the productivity and impact of the published work of a scientist or scholar. The index is based on the set of the scientist's most cited papers and the number of citations that they have received in other publications. The index can also be applied to the productivity and impact of a group of scientists, such as a department or university or country, as well as a scholarly journal. The index was suggested by Jorge E. Hirsch, a physicist at UCSD, as a tool for determining theoretical physicists' relative quality[1] and is sometimes called the Hirsch index or Hirsch number.

Definition and purpose[edit]

h-index from a plot of decreasing citations for numbered papers

The index is based on the distribution of citations received by a given researcher's publications. Hirsch writes:

A scientist has index h if h of his/her Np papers have at least h citations each, and the other (Np − h) papers have no more than h citations each.

In other words, a scholar with an index of h has published h papers each of which has been cited in other papers at least h times.[2] Thus, the h-index reflects both the number of publications and the number of citations per publication. The index is designed to improve upon simpler measures such as the total number of citations or publications. The index works properly only for comparing scientists working in the same field; citation conventions differ widely among different fields.

The h-index serves as an alternative to more traditional journal impact factor metrics in the evaluation of the impact of the work of a particular researcher. Because only the most highly cited articles contribute to the h-index, its determination is a relatively simpler process. Hirsch has demonstrated that h has high predictive value for whether a scientist has won honors like National Academy membership or the Nobel Prize. The h-index grows as citations accumulate and thus it depends on the 'academic age' of a researcher.

The h-index across disciplines and career levels[edit]

Hirsch suggested (with large error bars) that, for physicists, a value for h of about 12 might be typical for advancement to tenure (associate professor) at major research universities. A value of about 18 could mean a full professorship, 15–20 could mean a fellowship in the American Physical Society, and 45 or higher could mean membership in the United States National Academy of Sciences.[3]

The London School of Economics found that professors in the social sciences had average h-indices ranging from 2.8 (in law) to 7.6 (in economics).[4]

Among the 22 scientific disciplines listed in the Thomson Reuters Essential Science Indicators Citation Thresholds, physics has the second most citations after space science.[5] During the period January 1, 2000–February 28, 2010, a physicist had to receive 2073 citations to be among the most cited 1% of physicists in the world.[5] The threshold for space science is the highest (2236 citations), and physics is followed by clinical medicine (1390) and molecular biology & genetics (1229). Most disciplines, such as environment/ecology (390), have fewer scientists, fewer papers, and fewer citations.[5] Therefore, these disciplines have lower citation thresholds in the Essential Science Indicators, with the lowest citation thresholds observed in social sciences (154), computer science (149), and multidisciplinary sciences (147).[5]

Little systematic investigation has been made on how academic recognition correlates with h-index over different institutions, nations and fields of study. However, Hirsch estimates that after 20 years a "successful scientist" will have an h-index of 20, an "outstanding scientist" an h-index of 40, and a "truly unique" individual an h-index of 60. However, he points out that values of h will vary between different fields.[6]

For the most highly cited scientists in the period 1983–2002, Hirsch identified the top 10 in the life sciences (in order of decreasing h): Solomon H. Snyder, h = 191; David Baltimore, h = 160; Robert C. Gallo, h = 154; Pierre Chambon, h = 153; Bert Vogelstein, h = 151; Salvador Moncada, h =143; Charles A. Dinarello, h =138; Tadamitsu Kishimoto, h =134; Ronald M. Evans, h =127; and Axel Ullrich, h = 120. Among 36 new inductees in the National Academy of Sciences in biological and biomedical sciences in 2005, the median h-index was 57.[1]

Calculation[edit]

The h-index can be manually determined using citation databases or using automatic tools. Subscription-based databases such as Scopus and the Web of Knowledge provide automated calculators. Harzing's Publish or Perish program calculates the h-index based on Google Scholar entries. In July 2011 Google trialled a tool which allows scholars to keep track of their own citations and also produces an h-index and an i10-index.[7] Each database is likely to produce a different h for the same scholar, because of different coverage: Google Scholar has more citations than Scopus and Web of Science but the smaller citation collections tend to be more accurate. In addition, specific databases, such as the INSPIRE-HEP database can automatically calculate the h-index for researchers working in high energy physics.

A detailed study showed that the Web of Knowledge has strong coverage of journal publications, but poor coverage of high impact conferences. Scopus has better coverage of conferences, but poor coverage of publications prior to 1996; Google Scholar has the best coverage of conferences and most journals (though not all), but like Scopus has limited coverage of pre-1990 publications.[8][9] The exclusion of conference proceedings papers is a particular problem for scholars in computer science, where conference proceedings are considered an important part of the literature.[10] Google Scholar has been criticized for producing "phantom citations," including gray literature in its citation counts, and failing to follow the rules of Boolean logic when combining search terms.[11] For example, the Meho and Yang study found that Google Scholar identified 53% more citations than Web of Knowledge and Scopus combined, but noted that because most of the additional citations reported by Google Scholar were from low-impact journals or conference proceedings, they did not significantly alter the relative ranking of the individuals. It has been suggested that in order to deal with the sometimes wide variation in h for a single academic measured across the possible citation databases, that one should assume false negatives in the databases are more problematic than false positives and take the maximum h measured for an academic.[12]

Advantages[edit]

Hirsch intended the h-index to address the main disadvantages of other bibliometric indicators, such as total number of papers or total number of citations. Total number of papers does not account for the quality of scientific publications, while total number of citations can be disproportionately affected by participation in a single publication of major influence (for instance, methodological papers proposing successful new techniques, methods or approximations, which can generate a large number of citations), or having many publications with few citations each. The h-index is intended to measure simultaneously the quality and quantity of scientific output.

Criticism[edit]

There are a number of situations in which h may provide misleading information about a scientist's output:[13] (However, most of these are not exclusive to the h-index.)

Alternatives and modifications[edit]

Various proposals to modify the h-index in order to emphasize different features have been made.[22][23][24][25][26][27] As the variants have proliferated, comparative studies have become possible and they demonstrate that most proposals do not differ significantly from the original h-index as they remain highly correlated with it.[28]

See also[edit]

References[edit]

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  4. ^ Key Measures of Academic Influence
  5. ^ a b c d "Citation Thresholds". Science Watch. May 1, 2010. Retrieved 13 May 2010. 
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  21. ^ Cyril Labbe (2010) Ike Antkare one of the great stars in the scientific firmament, Laboratoire d'Informatique de Grenoble RR-LIG-2008 (technical report), Joseph Fourier University
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Further reading[edit]

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