Highly Cited Researchers™ from Clarivate™ is an annual list recognizing influential researchers in the sciences and social sciences from around the world. The 2021 list contains about 3,800 Highly Cited Researchers in 21 fields of the sciences and social sciences and about 2,800 Highly Cited Researchers identified as having exceptional performance across several fields. The list focuses on contemporary research achievement: only highly cited papers in science and social sciences journals indexed in the Web of Science Core Collection™ during the 11-year period 2010-2020 were surveyed. Highly cited papers are defined as those that rank in the top 1% by citations for field and publication year.
The data derive from Essential Science Indicators™ (ESI), a component of InCites™. The fields are also those employed in ESI – 21 broad fields defined by sets of journals and exceptionally, in the case of multidisciplinary journals such as Nature and Science, by a paper-by-paper assignment to a field based on an analysis of the cited references in the papers. This percentile-based selection method removes the citation advantage of older papers relative to recently published ones, since papers are weighed against others in the same annual cohort.
Researchers who, within an ESI-defined field, publish highly cited papers are judged to be influential, so the production of multiple top 1% papers is interpreted as a mark of exceptional impact. Relatively younger researchers are more likely to emerge in such an analysis than in one dependent on total citations over many years. To be able to recognize early and mid-career as well as senior researchers is one of the goals in generating the list of Highly Cited Researchers. The determination of how many researchers to include in the list for each field is based on the population of each field, as represented by the number of disambiguated author names on all highly cited papers in that field, 2010-2020. The ESI fields vary greatly in size, with Clinical Medicine being the largest and Economics and Business being the smallest in terms of researchers. The square root of the number of authors in each field indicated how many individuals should be selected.
Authors with the greatest number of highly cited papers in an ESI field at the threshold for inclusion and above are putative selectees. However, another criterion for selection is that the researcher must have enough citations to their highly cited papers to rank among all authors in the top 1% by total citations in the ESI field in which that person is considered. All who published highly cited papers at the threshold level and with citations in the field at the level of top 1% are admitted to the list, even if the final list then exceeds the number given by the square root calculation.
In addition, and as concession to the somewhat arbitrary cut-off, any researcher with one fewer highly cited paper than the threshold number is also admitted to the list if total citations to their highly cited papers ranks that individual in the top 50% by total citations of those at the threshold level or higher. The justification for this adjustment is that it seems to work well in identifying influential researchers, in the judgment of the Clarivate citation analysts.
Of course, there are many highly accomplished and influential researchers who are not recognized by the method described above and whose names do not appear in the 2021 list. This outcome would hold no matter what specific method were chosen for selection. Each measure or set of indicators, whether total citations, h-index, relative citation impact, mean percentile score, etc., accentuates different types of performance and achievement. Here we arrive at what many expect from such lists but what is unobtainable: that there is some optimal or ultimate method of measuring performance. The only reasonable approach to interpreting a list of top researchers such as ours is to fully understand the method behind the data and results, and why the method is used. With that knowledge, in the end, the results may be judged by users as relevant or irrelevant to their needs or interests.
The data used in the analysis and selection of Highly Cited Researchers derives from Essential Science Indicators (ESI), 2010-2020, which then included approximately 169,000 highly cited papers. Each of these papers ranked in the top 1% by total citations according to its ESI field assignment and year of publication. For more information on the identification of highly cited papers in Essential Science Indicators, see the ESI help file at Essential Science Indicators.
Essential Science Indicators surveys the Science Citation Index Expanded™ and Social Sciences Citation Index™ components of the Web of Science™, meaning journal articles in the sciences and social sciences. The analysis is further limited to items indexed as articles or reviews only and does not include letters to the editor, correction notices, and other marginalia.
In Essential Science Indicators, all papers, including highly cited papers, are assigned to one of 22 broad fields (the 22nd is Multidisciplinary, on which see below). Each journal in Essential Science Indicators is assigned to only one field and papers appearing in that title are similarly assigned. In the case of multidisciplinary journals such as Science, Nature, and others, a special analysis is undertaken. Each article in such publications is individually reviewed, including an examination of the journals cited in its references. The paper is then reclassified to the most frequently occurring field represented by the article’s cited references.
For each ESI field, author names are disambiguated through advanced clustering methods and the number of clusters is counted, each cluster representing a unique individual. Based on the number of clusters (individuals) for each field, the square root of that number is calculated. That number is used to decide approximately how many researchers to include in each ESI field. From the list of authors in a field ranked by number of highly cited papers, the number of papers at the rank represented by the square root score determines the threshold number of highly cited papers required for inclusion.
In addition, citations to an individual’s highly cited papers must meet the threshold for total citations used in the 2010-2020 (6th bimonthly) version of ESI for including a researcher in the top 1% (highly cited list) for an ESI field.
If an author has one fewer highly cited paper than this threshold, but citations to their highly cited papers ranks them in the top 50% by citations among those with highly cited papers at or above the threshold, these individuals are also selected.
|Citation to HCPs
|Field Paper Threshold
|Field Citation Threshold
|Field Citation Threshold if One Fewer Paper than Threshold Number
A criticism of past Highly Cited Researchers lists was that the methodology systematically neglected to identify researchers with cross-field influence: a researcher might contribute multiple highly cited papers in several different fields but would not register enough highly cited papers in any one field for selection. The criticism was valid and welcome. To find individuals with impact equivalent to those we select in a single field, we normalize the highly cited paper counts across fields, so that a paper in Clinical Medicine has the same ‘weight’ as one in Pharmacology/Toxicology. To do this we fractionate the count for each highly cited paper according to the threshold number used in each field. The fraction for a paper is larger in Pharmacology/Toxicology than in Medicine. If, after collecting all the highly cited papers of an author in all fields we find that the sum of the fractionated paper counts is 1 or more, this demonstrates that the individual has as much influence as those chosen in a single field. A similar procedure is employed for the citation counts, the second criteria for selection.
|Citation to HCPs
|Field Citation Threshold
|Field Paper Threshold
|Field Paper Score
|Field Citation Score
|Cross-Field Paper Score
|Cross-field Citation Score
The fictional researcher Joseph Savant published 15 highly cited papers in four ESI fields during the period 2010-2020. Seven papers in Field 6, with a threshold number of eight for selection, earned Savant a credit of .875 (or 7/8ths). Three papers in Field 14, with a threshold number of six for selection, were worth .5. The sum of the fractional paper counts in each field yielded a total Cross-Field paper score of 1.67. A score of 1 or more indicates that the individual achieved impact equivalent to a researcher chosen in a specific ESI field. The second criterion for selection as a Highly Cited Researcher is enough citations to rank in the top 1% by citations for a field. Again, citations in different fields were fractionated in a similar manner to the treatment of papers. In the example above, Professor Savant earned more than five times the number of citations needed for selection as an influential cross-field researcher.
To award credit to a single author among many tens or hundreds listed on a paper strains reason, so this year any highly cited paper with more than 30 authors or explicit group authorship was eliminated from our analysis.
Finally, we exclude retracted articles in our analysis of highly cited papers. Also, researchers found to have committed scientific misconduct in formal proceedings conducted by a researcher’s institution, a government agency, a funder or a publisher are excluded from our list of Highly Cited Researchers.
In 2019 we began to exclude authors whose collection of highly cited papers revealed unusually high levels of self-citation. For each ESI field, a distribution of self-citation was obtained, and extreme outliers (a very small fraction) were identified and evaluated. For a description of the methodology used to exclude authors with very high levels of self-citation, please see: Adams, J., Pendlebury, D. and Szomszor, M., “How Much is Too Much? The Difference between Research Influence and Self-Citation Excess,” Scientometrics, 123 (2):1119–1147, May 2020.