Record citations in 2011 contribute to maintenance of the impact factor of BJN

In late June 2012, the Institute for Scientific Information (ISI) released its annual statistics on citations of articles published in previous years in scientific journals. A number of different summary statistics are produced by the ISI, the most widely discussed being the impact factor. I have used previous editorials to keep readers informed of the most recent statistics for the BJN and to analyse them in relation to those of comparator journals and to temporal changes. The BJN is listed in the Nutrition and Dietetics category of ISI Journal Citation Reports. In 2011, there were seventy-two journals listed in this category, including review journals and journals in the areas of obesity (e.g. International Journal of Obesity, Obesity) and lipidology (e.g. Progress in Lipid Research, Lipids). The impact factor of a journal is calculated as the number of citations of papers published in the previous 2 years divided by the number of papers published in those 2 years. Thus, the impact factor for 2011 (issued in 2012) is based upon the number of citations during 2011 of papers published in a particular journal in 2009 and 2010 divided by the number of papers published in that journal in 2009 and 2010. Clearly, this favours very rapidly moving areas of research. Hence, journals such as Nature, Cell and Science have high impact factors (36·28, 32·40 and 31·20, respectively, for 2011). For the past 10 years, the two highest ranked journals in the Nutrition and Dietetics category have been Progress in Lipid Research and Annual Reviews in Nutrition, and with impact factors of 10·67 and 9·45, respectively, for 2011. Table 1 lists the impact factors for the BJN and nine comparator journals over the period 2001–11 inclusive. The comparator journals all publish a similar range of material as does the BJN, including molecular, cellular, whole body, human, clinical, public health and experimental animal nutrition and, in most cases, also farm animal nutrition. It is evident that the American Journal of Clinical Nutrition is firmly established as the highest ranked journal in this category that is not solely limited to publishing review articles. In 2011, the impact factor of the BJN remained almost the same as in 2010 (3·01 v. 3·07), representing 2823 citations in 2011 to the 937 articles published in 2009 and 2010. Maintaining an impact factor above 3 is important, keeping the BJN in the top 30 % of journals in this growing category. Readers may be interested in the impact factors of our sister journals: 4·84, 2·77 and 2·17 for Nutrition Research Reviews (ranked 4/72), Proceedings of the Nutrition Society (ranked 23/72) and Public Health Nutrition (ranked 34/72), respectively. Table 2 lists the articles published in the BJN during 2009 and 2010 that were most highly cited in 2011. This table indicates the importance of review and supplement articles, in addition to research papers, to the impact factor of the journal. Although the articles published in 2009 continue to be cited (Table 2), they will not contribute to the impact factor for 2012, which will be based upon articles published in 2010 and 2011. One argument against the importance of the impact factor in indicating the ‘value’ of a journal is that the time frame over which it is calculated is too short to really reflect the impact that the articles that a journal publishes will have. Thus, alternative measures of article citations are available. These include the total number of citations made to articles published in a journal, the 5-year impact factor and the cited half-life of articles. Table 3 lists the total number of citations made to articles published in the BJN, irrespective of their year of publication, during the years 2000–11. In 2011, articles published in the BJN were cited 15 036 times, placing the BJN fifth in the Nutrition and Dietetics category for total citations in 2011. The total number of citations of articles in the journal has increased year-on-year and increased by 7 % from 2010 and by over 170 % since 2000. The cited half-life of a journal (Table 3) is the median age of the articles published in that journal that are cited in the reporting year. Thus, publication of articles that remain important (or controversial) long after they are published will result in a long cited half-life. The cited half-life of the BJN for 2011 was 6·9 years, indicating that half of the citations to articles to the BJN in 2011 were to articles published in 2004 or before. Thus, it seems to me that the BJN is publishing articles that are seen as important in the short term, as judged by the reasonably high impact factor (within the journal category), but which remain important for many years, as judged by the cited half-life. For comparison, the cited half-lives for the American Journal of Clinical Nutrition and the Journal of Nutrition for 2010 were 8·2 and 8·9 years, respectively. The immediacy index is calculated as citations of articles published in the reporting year (e.g. 2011) by papers published in that same year. It is a measure of how immediately important (or controversial) published papers are. For 2011, the immediacy index of the BJN was 0·519 (245 citations in 2011 out of 472 articles published in 2011). In 2008, the 5-year impact factor was calculated for the first time; this is the number of citations in the year to articles published in the previous five years. For 2011, the 5-year impact factor of the BJN was 3·34 British Journal of Nutrition (2012), 108, 759–761 q The Author 2012