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Yoshihiro Yonekawa, MD Mass Eye & Ear / Boston Children's RETINA Roundup Editor Journal impact factors (JIFs) are numbers calculated to represent "how good" academic journals are. In this article, we're going to dig deeper and determine whether "how good" is really what JIFs measure. In any case, JIFs are numbers that authors and publishers alike keep an eye on, and use. So how do we use these numbers? Authors may consider a journal's IF when determining where to submit their latest and greatest paper. Some universities may look at the impact factors of the journals that their faculty are publishing in when promotions are being considered. Same with some academic job applications and grant reviews. There are also rankings of journals based on JIF, and publishers take this seriously. The JIF is therefore a big deal. However, like with most metrics that try to place a number on quality, there are controversies and limitations. Please read on to find out how these numbers are calculated, in order to get a good sense for what these numbers actually mean.
The JIF is the average number of times that a journal's papers published the past 2 years are cited in the assessment year. Thomson Reuters publishes this metric annually. They take the number of total citations in the assessment year, and divide it by the number of papers published the 2 years prior.
How Impact Factors are Calculated
For example, the 2017 JIF for our journal RETINA is 3.700. This means that papers published in RETINA in 2015 and 2016, on average, was cited 3.700 times, in 2017.This is slightly different from the 5-year JIF, which is the mean number of citations generated in the assessment year by papers published in the past 5 years. This therefore assesses the journal over a longer period of time, and most likely to show less variance. In general though, when people discuss JIFs, they are referring to the 2-year JIF. So JIFs are a useful metric to get a sense for the "citability" of journals in a field - not necessarily "how good." Let's see what precautions we need to take when looking at these numbers.