The Many Flavors of the Journal Impact Factor

For over 40 years, the Journal Impact Factor (JIF) has been the most talked-about metric in Journal Citation Reports. Developed by Dr. Eugene Garfield and Dr. Irving Shir in the 1970s, it was intended for use by librarians as a means to assess and curate their periodical collections. Over time, the use cases have expanded; JIF has become a valuable tool for publishers and for researchers as well.

JIF provides a ratio of citations to a journal in a given year to the citable items in the prior two years—in other words, it reflects the frequency that the average article in a given journal has been cited. JIF allows users to identify journals that, according to citations, are particularly impactful in their fields.

Over the years, Journal Citation Reports has adapted to market needs and publishing trends, and the JIF is no exception. While we have never changed the JIF calculation itself, we have added to the suite of citation-based metrics.

There are those in the world of citation metrics who feel that journal self-citations should be excluded from the calculation of performance metrics, and indeed, excessive self-citation can be grounds for journal suppression from the JCR. To offer a view into how self-citations influence JIF score, we included JIF without Self-Citations. While the contributions of journal self-citations were always listed in the JCR, in 2003, self-citation metrics were represented more explicitly, and when JCR moved to the InCites platform in 2014, Journal Impact Factor without Self-Citations received its own column in the Key Indicators table.

By comparing the JIF score with JIF without Self-Citations, users can see how a particular journal would fare if it didn’t cite itself; however, self-citation does not normally play a significant role in JIF scores. A 2002 report we produced showed that 82% of journals covered in the Science Citation Index edition of JCR had self-citation rates at or below 20%—the population showed a mean self-citation rate equal to 12.41, with a median of 9.04. More recently, our journal selection process essay states, “Among all journals listed in the 2014 JCR Science Edition, for example, 85% have self-citation rates of 15% or less. This shows that self-citation is quite normal for most journals.” Those journals that do self-cite to a degree that JIF scores can be impacted are reviewed for suppression from JCR, and their indexing in the Web of Science is also reviewed.

Particular fields are slow moving, meaning that citations are still actively increasing beyond the two-year time window of the JIF. Therefore, we added the 5-Year JIF to the JCR in 2008. This score is ideal for journals in those slower-moving fields that are characterized by a longer time to peak citation and/or a lengthier plateau of citation activity at the journal level. In the Linguistics category in 2016, 54% of all cited content was published before the year 2006, indicating that older articles continue to make an active contribution to the field. For the 180 journals in this category in 2016, over three-quarters have a higher 5-year Impact Factor than JIF, which uses a two year window.

Two years ago, with the aid of the late Dr. Garfield and his colleague Dr. Alexander Pudovkin, we developed the JIF Percentile, which allows a comparison of journals across different categories by providing a normalized score based on the rank of the JIF.

The JIF Percentile is the percentile of a journal’s JIF in relation to that of other journals in the same category. It is measured on a scale of 0 to 100, where the higher the value, the higher the Journal Impact Factor of the journal in relation to its peers. JIF Percentile is calculated as (n – r + .5)/n where n = the number of journals in the category, and r = the descending rank of the journal within that category. With the JIF Percentile, users can rationally compare the impact of journals across categories, such as the various subfields of Chemistry, Engineering, or Materials Science.

JIF is also expressed as a quartile, which helps measure the distribution of data. Quartiles in JCR are defined as follows:

X = the journal rank in category according to the metric (Journal Impact Factor, Total Citations, etc.)
Y = the number of journals in the category.
Z = Percentile rank (X/Y)

Q1: 0.0 < Z ≤ 0.25
Q2: 0.25 < Z ≤ 0.5
Q3: 0.5 < Z ≤ 0.75
Q4: 0.75 < Z

In the Compare Journals feature on JCR, users can see how each journal ranks within a given quartile, compared with other journals of their choosing, so long as each is categorized within the same subject. The ability to select multiple quartile metrics simultaneously allows for a comprehensive view of how each journal ranks within metrics for a given year.

The Web of Science features JCR metrics in its journal overlay, which supplies the JIF, 5-year JIF, rank in category, and quartile. InCitesBenchmarking & Analytics makes the following metrics available for your analytics needs: JIF, 5-Year JIF, JIF without Self-Citations, Article Influence Score, Eigenfactor, and Immediacy Index. In its most recent update InCitesadded JIF Quartile metrics, allowing users to view the distributions of their publications by JIF Quartile, and graphing publication trends over time.

The JCR is more than just the JIF and its variations. While the formula for calculating JIF is simple and while it gets most of the community’s attention, JCR includes a complex and textured set of data about the citation environment in and around a journal, as well as other citation-based metrics. Clarivate Analytics offers short-, medium-, and long-term analysis of journal performance in the JCR. It has the Immediacy Index, the Citing and Cited Half-livesEigenfactor metrics, and Article Influence Score. It also offers subject-level normalizations for many of these metrics. Whatever your journal evaluation needs, we have a metric for that!

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