Journal Impact Factor: What is it and How is it Calculated?
An offshoot of citation analysis is Journal Impact Factor (JIF) that is used to sort or rank journals by their relative importance. The underlying assumption behind Impact Factors (IF) is that journals with high IF publish articles that are cited more often than journals with lower IF.
Impact factors may be used by:
Where to find Journal Impact Factors?
The most notable source for journal impact factors is the annual publication called the Journal Citation Reports (JCR) published by Thomson Reuters.
How is the Journal Impact Factor Calculated?
Thomson defines impact factor as, “The journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the JCR year. The Impact Factor is calculated by dividing the number of citations in the JCR year by the total number of articles published in the two previous years. An Impact Factor of 1.0 means that, on average, the articles published one or two year ago have been cited one time. An Impact Factor of 2.5 means that, on average, the articles published one or two year ago have been cited two and a half times. Citing articles may be from the same journal; most citing articles are from different journals.”
A journal's impact factor for 2015 would be calculated by taking the number of citations in 2015 to articles that were published in 2014 and 2013 and dividing that number by the total number of articles published in that same journal in 2014 and 2013. Below is how Thomson calculated the 2015 impact factor for the journal Academy of Management Review :
Thus, the Journal Impact Factor of 7.288 for the journal, Academy of Management Review for 2015 indicates that on average, the articles published in this journal in the past two years have been cited about 7.288 times.
Things to Consider when calculating Impact Factor:
Publication Date: The impact factor is based on citation frequency of articles from a journal in their first few years of publication. This does not serve well the journals with articles that get cited over a longer period of time (let's say, 10 years) rather than immediately. In other words, journals in rapidly expanding fields such as cell biology and computing tend to have much higher immediate citation rates leading to higher IFs than journals in fields like Education or Economics.
Journal Impact Factor not Article Impact Factor: Citations to articles in a journal are not evenly distributed. In fact, some articles in a journal may not be cited at all but a few highly cited articles could lead to a high IF. Therefore, the IF does not accurately reflect the quality of individual articles published in a journal. Also, journals with more issues and articles can have higher Impact Factors which could be misleading as it does not really reflect the quality of articles.
Review Articles: Review articles (which tend to receive more citations), editorials, letters, and news items are not counted in article total but if cited are counted as citations for the journal. This leaves room for manipulation of ratio used to calculate impact factors leading to inflated impact factors in some cases.
Clinical Journals: Clinical journals usually have low citation counts. This puts such journals at a disadvantage with research journals in the field that have higher citation counts.
Uneven Coverage: The Journal Citation Reports focuses much more on disciplines where the primary means of publishing is through journal article. It provides less coverage to areas in Social Sciences and Humanities, where books and other publishing formats are more prevalent.