The Truth Around the h Index
What is the h-Index?
In 2005, Dr. Jorge E. Hirsch of the University of California, San Diego, suggested the h-Index as a tool for assessing the published work of theoretical physicists, creating an author-level metric that attempts to blend the measurement of a researcher’s productivity and citation impact. Since then, the metric has spread beyond physics to every field of research.
In Hirsch’s words:
“For the few scientists that earn a Nobel Prize, the impact and relevance of their research work is unquestionable. Among the rest of us, how does one quantify the cumulative impact and relevance of an individual’s scientific research output? In a world of limited resources, such quantification (even if potentially distasteful) is often needed for evaluation and comparison purposes.” (J.E. Hirsch, “An index to quantify an individual’s scientific research output,”
PNAS, 102 : 16569-72, 2005).
The h-Index can be applied to any level of aggregation and can reveal information about how the citations are distributed over a set of documents. At the author level, it is considered to be an indicator of a researcher’s lifetime scientific achievements. Some clear advantages of the h-index are that it is mathematically simple; it favors authors who have produced a large quantity of high-impact papers instead of rewarding those whose output, while possibly prolific, is demonstrably less impactful; and it is not artificially inflated by the presence of one or two highly cited publications.
How is the h-Index calculated?
The h-index is based on a list of publications ranked in descending order by the times cited. The value of h is equal to the number of papers (N) in the list that have N or more citations. For example, an h-index of 12 indicates that in the dataset, 12 papers were cited at least 12 times each.
The h-Index is a time-dependent measure since it is directly proportional to the length of a researcher’s career and how many articles he or she has published. Early-career researchers are at a disadvantage when compared to more senior researchers, because the latter have had more time to produce work and accrue citations.
Additionally, the h-Index can be very different across disciplines due to the variation in the average citation rates of different fields. Moreover, the h-Index also does not exclude self-citations.
If the h-Index is a relatively simple mathematic index, why does an author’s h-Index sometimes vary according to different sources?
The h-Index will vary based on the underlying database used to calculate the index, as a result of the different coverage of journals and the papers published in them. With coverage back to the 1900s, Web of ScienceTM covers the full length of any current researcher’s career, providing the most accurate h-Index.
"The h-Index measures the broad impact of an individual's work, avoids all of the disadvantages of the criteria listed above, and usually can be found very easily by ordering papers by 'times cited' in the Thomson ISI Web of Science database." (J.E. Hirsch, “An index to quantify an individual’s scientific research output,” 102 : 16569-72, 2005).
Put those calculators away!
Get an author’s h-Index directly in Web of Science. Search by author and select the Citation Report to view the calculated h-Index. As we know, the h-Index will vary based on the dataset as well as the timespan. For example, when looking at the h-Index with a Web of Science subscription from 2001 on, the h-Index is 23 but adding in all-time raises it to 27.