Temporal Trends (1999–2015) in the Impact Factor of Biomedical Journals Published by US and EU Scientific Societies

OBJECTIVE: The impact factor has emerged as the most popular index of scientific journals’ resonance. In this study we aimed to examine the impact factor trends of journals published by scientific bodies in the United States of America (USA) and Europe (EU). METHODS: We randomly chose 11 categories of Journal of Citation Reports and created three research classes: clinical medicine, laboratory medicine, and basic science. The impact factor values for the years 1999–2015 were abstracted, and the impact factor of US and EU journals was studied through the years. RESULTS: A total of 265 journals were included in the final analysis. The impact factor of US journals was higher than that of EU journals throughout the study period. In addition, for both US and EU journals the median impact factor increased throughout the study period. The rate of annual change in the impact factor throughout the study period was lower for US than EU journals (1.85% versus 3.55%, P=0.019). A higher median annual increase was seen in the impact factor during the period 1999–2008 compared to the period 2009–2015 for both US (P<0.001) and EU (P=0.001) journals. In fact, during the second period the US median impact factor value did not show significant changes (P=0.31), while the EU median impact factor continued to increase (P<0.001). CONCLUSION: The impact factor of EU journals increased at a significantly higher rate than and approached that of the US journals during the last 16 years.