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Dissecting the 2018 Journal Impact Factor

The new journal impact factor (JIF) has just been released by Clarivate Analytics. Once again, ACS Energy Letters recorded an impressive gain with a new JIF of 16.331. This puts ACS Energy Letters among the leading energy research journals. Of the 12515 journals in all disciplines indexed by Clarivate Analytics, ACS Energy Letters now ranks among the top 125 journals. It is ranked #1 in Electrochemistry category and among the top 5 in the categories of Chemistry/Physical, Energy & Fuels, and Nanoscience and Nanotechnology. The immediacy index, which reflects citations for published articles during the same year (viz. 2018), was 3.896. Thanks to our authors, readers, and reviewers for helping us build a strong scientific journal spanning multidisciplinary topics related to energy conversion and storage. It is their contributions that helped us to gain this impact in just 3 years. The JIF published in June 2019 reflects the 2018 citations of research articles published during the previous 2 years (2016− 2017). ACS Energy Letters published 531 articles and reviews in 2016 and 2017 and received 8672 citations of them in 2018. It is important to note that Clarivate Analytics includes citations to editorial matter, but it does not count them as published articles. This is in contrast to Scopus CiteScore (https://www.scopus.com/ sources), which includes both research articles and editorial matter fully in their calculation of CiteScore. Hence, many popular journals that register relatively high JIFs do not reflect the same impact in CiteScore analysis. (In the latest SCOPUS analysis, ACS Energy Letters registered a CiteScore of 13.92.) While the JIF provides a metric to gauge the overall impact of the journal, it fails to convey the impact of individual papers. Hence, one needs to see a detailed analysis of citations of individual papers in each journal. Figure 1 shows the distribution of 2018 citations of ACS Energy Letters papers published in 2016−2017. Other journals also exhibit such a citation distribution in the JCR analysis. Interestingly, a small percent of the papers garner a disproportionately higher number of citations (see, for example, the impact of the top 10% papers in Table 1). Reviews typically draw more citations than regular research articles. For the journals presented in Table 1, the overall article median (50 percentile papers) is less than the JIF, while the review median is greater than the JIF. This suggests that more than 50% of the published research articles carry less impact than the JIF. Even in high impact journals, a significant fraction of papers fail to gain citations. For example, for the journals listed in Table 1, the impact of the bottom 10% papers, which range from 0 to 3.89, fail to correlate with the JIF trend. The dissection of the impact factor as noted in Table 1 highlights why one should not equate the journal impact factor to the impact of published articles. Another element that accounts for the JIF is the popularity of the field and number of researchers around the world engaged in that topic. For example, journals that publish energy and materials related content have seen a significant rise in JIF in recent years. This topic-dependent citation impact points out the ambiguity of comparing the numerical value of JIF while evaluating journals across different disciplines We also analyzed the impact of a few key topics in ACS Energy Letters using the Web of Science Core Collection (Clarivate Analytics) database. The individual impact of these topics in terms of published research articles and reviews (2016−2017) and their citations in 2018 is compiled in Table 2.