On 17 June 2010, the Institute for Scientific Information 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 Institute for Scientific Information, 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(Reference Calder1–Reference Calder5). The BJN is listed in the Nutrition and Dietetics category of Institute for Scientific Information Journal Citation Reports®. In 2009, there were sixty-six 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 2009 (issued in 2010) is based upon the number of citations during 2009 of papers published in a particular journal in 2007 and 2008 divided by the number of papers published in that journal in 2007 and 2008. Clearly, this favours very rapidly moving areas of research, and so journals such as Nature, Cell and Science have high impact factors (34·45, 31·15 and 29·75, respectively, for 2009). For the past 8 years, the two highest ranked journals in the Nutrition and Dietetics category have been Annual Reviews in Nutrition and Progress in Lipid Research, with impact factors of 8·78 and 8·17, respectively, for 2009. Table 1 lists the impact factors for the BJN and nine comparator journals over the period 2001–9 inclusive. The comparator journals all publish a similar range of material as does the BJN, including molecular, cellular, whole body, human, clinical, public health, 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 2009, the impact factor of the BJN rose from 2·76 to 3·45 (2351 citations in 2009 to the 684 articles published in 2007 and 2008), a rise of 25 % which I take to indicate the health of the journal. For 2009, the impact factors of our sister journals were 4·31, 2·75 and 1·59 for Proceedings of the Nutrition Society (ranked 5/66), Public Health Nutrition (20/66) and Nutrition Research Reviews (41/66), respectively.
Table 1 Impact factor of the British Journal of Nutrition and comparator journals over the period 2001–9*
* Data are from Institute for Scientific Information Journal Citation Reports®.
† Ranking amongst journals in the Nutrition and Dietetics subject category is shown in parentheses beside each impact factor.
Table 2 lists the articles published in the BJN during 2007 and 2008 that were most highly cited in 2009. This table indicates the importance of review articles and the Horizons in Nutritional Science series to the impact factor of the journal. Although the articles published in 2007 continue to be cited (Table 2), they will not contribute to the impact factor for 2010 which will be based upon the articles published in 2008 and 2009.
Table 2 Articles published in British Journal of Nutrition in 2007 and 2008 that were most highly cited in 2009*
* Data were obtained from Institute for Scientific Information Web of Science® on 22 June 2010.
An 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 2000–8. In 2009, the articles published in the BJN were cited 12 904 times, placing the BJN fifth in the Nutrition and Dietetics category for this statistic. It is apparent that the total number of citations of articles in the journal has increased year-on-year, and increased by 14 % from 2008 and by over 130 % 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 2009 was 7·0 years, indicating that half of the citations to the articles in the BJN in 2009 were to the articles published in 2002 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 2009 were 7·5 and 7·6 years, respectively. The immediacy index is calculated as citations of articles published in the reporting year (e.g. 2009) by papers published in that same year. It is a measure of how immediately important (or controversial) published papers are. For 2009, the immediacy index of the BJN was 0·530 (255 citations in 2009 to the 481 articles published in 2009). In 2008, the 5-year impact factor was calculated for the first time; this is the number of citations in the year to the articles published in the previous 5 years. For 2009, the 5-year impact factor of the BJN was 3·57 (5464 citations in 2009 to the articles published in 2004–8 inclusive), placing it 15th in the Nutrition and Dietetics category. For comparison, 5-year impact factors for the American Journal of Clinical Nutrition and the Journal of Nutrition for 2009 were 7·74 and 4·54, respectively. The final statistic shown in Table 3 is the Eigenfactor™ score. This is a complex calculation which, like impact factor, is a ratio of the number of citations to the total number of articles published. However, unlike the impact factor, the Eigenfactor™ score counts citations to journals in both the sciences and social sciences, eliminates self-citations (i.e. every reference from one article in a journal to another article from the same journal is discounted) and weights each reference according to a stochastic measure of the amount of time that the researchers spend reading the journal(Reference Bergstrom6). For 2009, the Eigenfactor™ score of the BJN was 0·0308, placing it fifth in the Nutrition and Dietetics category. For comparison, Eigenfactor™ scores for the American Journal of Clinical Nutrition and the Journal of Nutrition for 2009 were 0·09 393 and 0·06 086, respectively.
Table 3 Citation statistics for the British Journal of Nutrition 2000–9
Another relatively new statistic is the Article Influence™ score, which calculates the relative importance of the journal on a per-article basis. It is the journal's Eigenfactor™ score divided by the fraction of articles within the category published by that journal. That fraction is normalised so that the mean Article Influence™ score within the category is 1·00. A score greater than 1·00 indicates that each article in the journal has above-average influence, while a score less than 1·00 indicates that each article in the journal has below-average influence. For 2009, the Article Influence™ score of the BJN was 0·964, placing it 14th in the Nutrition and Dietetics category. For comparison, Article Influence™ scores for the American Journal of Clinical Nutrition and the Journal of Nutrition for 2008 were 2·211 and 1·216, respectively.
My overall view based upon these statistics is that the BJN is doing well, but could do better. As I indicated in my previous editorials(Reference Calder2–Reference Calder5, Reference Calder7), the BJN is receiving more submissions and is publishing more articles than ever before(Reference Calder7). This suggests that the journal is in very good health and is viewed favourably by researchers within the discipline. My aim is to act to further improve the impact factor, the 5-year impact factor and the Article Influence™ score in order that the prestige and attractiveness of the BJN are maintained in the face of mounting competition from other journals, and that its perceived quality is enhanced. An improvement in (perceived) quality of the BJN will assure its place among the top journals in the field.
