Impact factor (IF) of an academic journal refers to the average number of citations to the recently published articles, i.e. published in the previous two years, in the journal. These articles include research papers, reviews and news articles.
In the field of research, journals, indexed in Journal Citation Reports, with more IF are considered to be more important as compared to the lower ones.
However, impact factor is not the clear indication of the citations to the specific papers or of the further work of the individual researchers. (Although, impact factor increases the value of the paper on the bases of citations but often the citations are from the secondary sources, so that the findings are often credited to the secondary sources resulting in less impact of the original research author.)
In the figure below, you can see that the impact factor correlates well with the five-year median of citations to primary research papers published in 2008–2012. Median, here, refers to the “minimum number of citations received by half of the papers, and thus is robust to outliers and variations in the shape of the distribution.”
According to the Editorial of the Nature Materials, some journals in this graph generate a inconsistent quantity of non-primary articles such as The Lancet and The Journal of the American Medical Association, and some journals have significantly changed the amount of content produced per year such as PLoS ONE, the median number of citations for which is 1 while it’s 2011 IF is 4.1 that has been attributed to the large increase of contents produced from 2008, when less than 3,000 papers were launched, to 2012, when about 19,000 papers were launched.
There is another aspect of research in the increase in the impact factor of the high impact journals i.e. The Matthew Effect or “the rich get richer”. People usually cite the popular articles by the well-known researchers either because they know their research or the authority know their research. Those well-known researchers in-turn incline towards the well-known journals resulting in more impact.
Hadas Shema, (2013). What’s wrong with citation analysis? Scientific American.
Editorial (2013). Beware the impact factor Nature Materials, 12 (2), 89-89 DOI: 10.1038/nmat3566