In the world of research publishing, a loose hierarchy of journals exists both overall and within individual fields. In science publishing generally, a handful journals are widely regarded as the most prestigious, such as Science and Nature. Within medicine, The New England Journal of Medicine, The Lancet and JAMA are among those considered top journals. While there is no official, agreed-upon list ranking journals by prestige, one objective feature commonly used to assess a journal is its “impact factor.” The impact factor represents how frequently, on average, an article in that journal has been cited in another paper in a particular year. Thomson Reuters calculates the impact factor for each journal annually in their Journal Citation Reports.
Deeper dive
The expectation is that the more frequently a journal’s research is cited, the stronger, more reliable and more impactful the science in that journal is, and the more highly the publication is regarded. That said, impact factor is not a perfect measure of journal quality and certainly not of individual papers in the journal. An infamously fraudulent retracted study that attempted to link autism to the MMR (measles-mumps-rubella) vaccine was published in The Lancet, by all accounts a prestigious journal with a high impact factor (39.207 in 2013-2014).
Some critics have also pointed out that a journal’s impact factor can be strongly influenced by only a small proportion of papers, which reveals little about all the other papers published in that journal but cited far less often. For example, a 2005 editorial in Nature notes that 89 percent of Nature’s impact factor the previous year was generated by just a quarter of the paper’s published in the preceding years. Finally, a New York Times op-ed by the creators of Retraction Watch notes that journals with a higher impact factor tend to have more retractions than those with lower impact factors, for unknown reasons.
While imperfect, the impact factor still provides a rough guide as to a journal’s reliability, which can be helpful if a reporter comes across a study published in an unfamiliar journal. Several sites provide ways to search for impact factors. The International Scientific Institute offers an index at SciJournal.org. Affiliates of universities, such as the University of Virginia Health System, sometimes include the impact factor in their listings.
The calculation is the ratio of A over B, in which A represents the number of times all items published in a particular journal during two consecutive years were cited in the third consecutive year, and B represents the total number of “citable items” published in that journal during those two consecutive years. Citable items typically refer to studies, reviews, proceedings or notes but not editorials or letters to the editor. For example, in 2013-2014 Nature had an impact factor of 42.351. The New England Journal of Medicine’s factor for that period was 54.42, and for JAMA it was 30.387. Other journals generally have impact factors in the lower single digits.