Data from citations provide a window into the impact of specific articles and authors, as well as the return on a country’s scientific investment (Narin and Hamilton 1996). Research embodied in publications also impacts invention through patent citations, which will be covered in the forthcoming Indicators 2022 report “Invention, Knowledge Transfer, and Innovation.”

Publications with more citations are considered to have more impact (Garfield 1955; Waltman, van Eck, and Wouters 2013).^{​Publications include some peer-reviewed journal articles that receive citations as examples of poor or outdated research, but often the articles refuting the low-quality article receive many more citations (Wallin 2005).} A small subset within the count of cited articles is categorized as highly cited articles (HCA), or the publications most frequently cited in other articles. This report presents data on the top 1% of HCA. The index of a country’s number of HCA relative to the world’s HCA (i.e., the HCA score) creates a comparable cross-country measure.^{​The share of S&E articles in the top 1% of cited articles is computed by field because different fields of science have different rates of citation. Details provided in Science-Metrix (2021a).} Conference proceedings are excluded from the HCA count because proceedings are not cited as often as journal articles.^{​Table SPBS-73 and Table SPBS-89 provide the data to compare the computer and information sciences publications in the top 1% for both journal articles and conference proceedings with the HCA scores with only journal articles. For the large producing countries, the HCA that include conference proceedings show the same trend but with slightly higher HCA scores as the HCA with only journal articles. This comparison is made for computer and information sciences because conference proceedings are close to 20% of the references in which proceedings are much less cited (Lisée, Larivière, and Archambault 2008).} In addition, conference papers are often later converted into articles in journals, and then it is the journal article that receives the majority of the citations. A minimum of 2 years from the publication date is required for calculation of HCA. This delay provides an opportunity for articles to accumulate citations after the publication (Wang 2012). This report provides data for articles published through 2018 because citations are calculated based on data up to 2020.

The United States contributed more than the expected number of articles to the world’s pool of HCA in 2018 (HCA score of 1.8) (Figure PBS-7).^{​The share of top 1% articles produced by a country is computed as follows: Sx = HCAx / Ax, where Sx is the share of output from country x in the top 1% most-cited articles; HCAx is the number of articles from country x that are among the top 1% of most-cited articles; and Ax is the total number of articles from country x with a relative citation (RC) score. Both HCAx and Ax are based on whole counting. The RC score is a normalized citation score assigned to a paper and used to rank articles into the top 1%. The RC score takes into consideration the citation behavior between fields and years of publication.} China’s HCA score is above the expected share of HCA given their level of 2018 articles (HCA score of 1.2). The HCA scores for China track its emergence as a major S&E research country; the HCA for China in 1996 was 0.3. Not all major article-producing countries received above the expected HCA score; Japan and India each had HCA scores less than 1.0 in 2018 (both with a score of 0.9).

The trends in HCA scores provide an opportunity to understand the changes in a country’s research impact. From 2008 to 2018, the impact of Chinese researchers grew faster than the impact of researchers in the United States and other countries (Conte et al. 2017; Xie and Freeman 2018). The HCA score of the United States was 2.0 for 2008 and 1.8 for 2018. Over the same decade, China’s HCA score more than doubled (from 0.5 in 2008 to 1.2 in 2018). The HCA scores for articles with authors in India and Japan also increased from 2008 to 2018; India’s HCA score increased from 0.7 to 0.9, and Japan increased from 0.6 to 0.9.

Changes over time in the HCA by field of science provide a more nuanced story of research across countries. The normalization of the HCA by output and scientific field using fractional counting spotlights countries with relatively low publication output but consistently high rankings among the most cited articles in a field:^{​Computing the HCA score using fractionally counted publications gives a more exact view of performance for countries with relatively fewer articles.} for example, agricultural sciences in Ireland (2.51), engineering in Singapore (2.87), and materials science in the Netherlands (2.43) (Table SPBS-59, Table SPBS-64, Table SPBS-67).