Evaluation based on scientific publishing: H-index


The H-index is also known as the Hirsch index and Hirsch number. The H-index was developed in 2005 by profesor Jorge E. Hirsch in University of California. The original article by Hirsch is available at http://arxiv.org/PS_cache/physics/pdf/0508/0508025v5.pdf.

The H-index is an easily computable index, which gives an estimate of the importance, significance and broad impact of a scientist's cumulative research contributions. It accounts for both productivity and impact of a scientist. It avoids most of the disadvantages of the other single-number metrics commonly used to evaluate scientific output of a researcher. With the value of the H-index being discipline and time-dependent, without corresponding standardization the H-index should be used to compare the relative importance of scientists only if they are of similar (scientific) age and work in similar disciplines. Shortcomings of the H-index are its inability to differentiate between active and inactive scientists, its weakness to differentiate between significant works in the past and the works which continue to be cited, and the difficulty to compare authors in different disciplines.


Calculation of the H-index

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.

H-index for journals

A journal has an h-index of h if there is at least h papers published in that journal that have received at least h citations each.

The calculation of the H-index is not dependent on the defined citation window. If the H-index is calculated for the whole lifetime of a journal, it favours journals that have been established a long time ago. For most uses such a journal H-index does not seem useful. Instead, one must decide on the most suitable citation window. An advantage of using the H-index instead of the Impact Factor for evaluating journals is that the impact gap between multidisciplinary journals, such as Nature and Science, and other journals is significantly reduced making comparisons meaningful between these journals. Additionally, the impact of journals specializing in review articles is inherently deflated because of the limited number of annual articles in such journals.

The h-index is an alternative to the IF when evaluating the impact of journals based on citation information. It gives a rough estimate on the productivity of a journal over a long time period. The good qualities of the h-index, compared with IF, are its stability and its balanced combining of quantity and quality, which reduces the over-valuing of journals that publish review articles. Because the h-index is not based on the average number of article citations, it is not influenced by the fact that a few articles might have massive amounts of citations. However, the amount of articles a journal has published will affect somewhat the h-index score. A journal that publishes many articles will most likely get a higher h-index score than a journal that does not publish as much, because each article is a potential subject for citing. A journal that does not publish many articles will probably not get a high h-index score even if most of its articles were to be widely cited. The h-index is not calculated for one specific fixed time period like the IF, but it can be calculated for whatever time period you want, for example, for one or five years. This allows comparisons of journals that differ from each other in terms of how fast the journals start getting citations. However, even with the h-index, it is still preferable to compare journals from within the same discipline.

Publish or Perish calculates the h-index of journals using citation data from Google Scholar. This opens up the possibility of evaluating the impact of a journal that is not part of the Thomson Reuters' Web of Science database, and therefore has no IF score. Since Google Scholar has wider ranging reference data than the Web of Science, it is able to give a fairer evaluation to journals that are cited mainly in sources outside the Web of Science, like books and journals that are not included in the Web of Science. In addition the h-index for journals is available on the SCImago Journal & Country Rank website.

Modifications of the H-index

  • Sidiropoulos et al. (the original article is available at http://www.springerlink.com/content/u8g072hw8842415l/fulltext.pdf) present several modifications of the original H-index: the contemporary H-index takes into account the age of the articles a scientist has published which makes it possible to distinguish between active scientists and those senior scientists that are no longer active. The trend H-index takes into account the age of the citations to each article which makes it possible to distinguish trendsetters i.e., scientists whose work is considered pioneering and continue to be cited. The normalized H-index makes it possible to compare scientists who have published different numbers of articles. The yearly H-index and normalized yearly H-index are indices for ranking journals and conferences.
  • The m-index is a scientist's H-index divided by the number of years since his/her first paper.
  • The a-index is the mean number of citations for the articles used to calculate the H-index.
  • The Individual H-index accounts for the differences among disciplines. It allows a less biased comparison of different research fields due to the consideration of co-authorship effects. Detailed description in Batista et al. at http://www.springerlink.com/content/a41351w08632151v/fulltext.pdf.

Calculation of H-index for Jorge E Hirsch in Web of Science database. The h-index is based on a list of publications ranked in descending order by the Times Cited. The H-index is indicated as a green line.
Image source: Web of Knowledge (Thomson Reuters) <http://www.isiknowledge.com/> 6.11.2009.

Factors influencing the H-index

There are differences in typical values of the H-index in different fields, determined in part by the average number of references in a paper in the field, the average number of papers produced by each scientist in the field, and the size (number of scientists) of the field. Subfields with typically large collaborations will exhibit larger values of the H-index.

Since values of H-index increase over time, it is apparent that a scientist's H-index depends on the person's scientific age, and the H-index always puts newcomers at a disadvantage and older, well-established scientists at an advantage.

H-index does not differentiate citations according to the citing journal and does not take into account the context of citations.

The H-index is strongly affected by the Total number of papers, which may underestimate scientists with short careers and scientists who have published only a few although significant papers.

H-index is insensitive to one or several outstandingly highly cited papers.

H-index is relatively insensitive to self-citations, because all self-citations to papers with less than H citations are irrelevant, as are the self-citations to papers with many more than H citations.

The H-index is strongly affected by the Total number of papers, which may underestimate scientists with short careers and scientists who have published only a few although significant papers.

Waltman & van Eck (2009) report that the H-index suffers from the problem of inconsistency, which cannot be solved by introducing yet another variant of the H-index, because the problem is shared by all H-index variants. Read the whole article at http://publishing.eur.nl/ir/repub/asset/16556/ERS-2009-043-LIS.pdf

Examples of H-index values for scientists