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NRF Rating Preparation: Author impact (h-index)

This guide provides information and tools for measuring research impact, focusing on the use of citation metrics in NRF rating preparation

Definition of the h-index

The h-index was developed by Professor Hirsch in 2005, a physicist at the University of California in San Diego, to qualify the impact and quantity of an individual's research performance.  The h-index paper was published in the Proceedings of the National Academy of Sciences of the United States of America 102 (46): 16569-16572. It reflects the productivity of authors based on their publication and citation records. 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. The total number of papers does not account for the quality of scientific publications, while the total number of citations could be affected by a large number of citations from a single publication of major influence, or having many publications with few citations each.

The h-index is intended to measure simultaneously the quality and quantity of scientific output. The index is therefore a measure of the number of high impact papers a scientist has published - the larger the number of influential papers, the higher the h-index, regardless of where  the work was published. It measures impact as well as quantity.

 

The H-index formula

A researcher has index h if h of his or her NP (number of papers) have at least h citations each and the other (NP - h) papers have fewer than h citations each.

For example, an h-index of 10 means that the researcher has 10 papers each of which has been cited at least 10 times in a subject discipline.

Different citation patterns could mean that researches that publish in the medical field could have  much higher h-index values than for instance a world class mathematician, or researches that publish in the Arts and Humanities.

When comparing researchers, all h-index values need to be found by using the same databases and the same methods.

The h-index may be less useful in some disciplines, particularly in the Arts and Humanities. 

Reporting citation metrics

Reporting citation metrics

Reporting Altmetrics

Advantages of the h-index

The h-index aims to address the main disadvantages of other bibliometric indicators, such as the total number of papers or the total number of citations:

  • It relies on citations to individual papers for quality, rather than journals
  • It is not skewed by a single well-cited, influential paper
  • It is not artificially inflated by a large number of poorly cited papers
  • High impact papers count regardless of publication in the top journals
  • It is a measure of comparison of scientists within a subject field
  • It is a measure for comparison for individuals,departments, programmes or a group of scientists
  • The h-index is intended to measure simultaneously the quality and quantity of scientific output.

Challenges of the h-index

While the h-index lends itself to measurement in the pure sciences, citation metrics in the Social Sciences, Arts and Humanities will always be underestimated, due to the very nature of the disciplines.

Weaknesses of the h-index:

  • It does not account for the numbers of authors in a paper
  • It does not account for the typical number of citations in different fields
  • It discards the information contained in author placement, where there is more than one author
  • It is bound by the total number of publications. This means that scientists with a short career are at an inherent disadvantage, regardless of the importance of their research
  • It does not consider the context citations
  • It gives books the same count as articles, making it difficult to compare scholars in fields that are more book oriented, such as the Humanities.