Journal Impact Factor
Can apply to: Journals indexed in the Clarivate Science Citation Index Expanded and/or Social Sciences Citation Index
Metric definition: The Journal Impact Factor is a measure reflecting the annual average (mean) number of citations to recent articles published in that journal. An essay written by the Institute of Scientific Information (ISI), now Clarivate Analytics, states “The JCR provides quantitative tools for ranking, evaluating, categorizing, and comparing journals. The impact factor is one of these; it is a measure of the frequency with which the “average article” in a journal has been cited in a particular year or period. The annual JCRimpact factor is a ratio between citations and recent citable items published.”
Metric calculation: From Wikipedia: “In any given year, the impact factor of a journal is the number of citations received in that year by articles published in that journal during the two preceding years, divided by the total number of articles published in that journal during the two preceding years.” Citations are counted for all items in the journal, though the citations are divided by only the number of “citable items” within the journal, as defined by the creators of the Journal Citation Report (currently Clarivate Analytics). Citable items are restricted by document type to articles and reviews.
Data sources: Citation data used to calculate journal citations is sourced all citations found in the Web of Science Core Collections
Appropriate use cases: The JIF can be useful in comparing the relative influence of journals within a discipline, as measured by citations. Used appropriately and in conjunction with other metrics, the JIF can be useful in collection development decisions made by librarians. As with all metrics, the JIF should be presented with appropriate context.
Limitations: The JIF has been published annually since 1975, and an extensive literature is available on its characteristics, limitations, and common misunderstandings related to its use. Some commonly noted limitations of the JIF:
- There is debate about the degree of transparency related to guidelines for the journals and article types selected for inclusion. Some claim there is not enough transparency while others feel that Clarivate’s documentation on the selection process is sufficient.
- A related debate surrounds the differing scope of the numerator and denominator used for calculating the Journal Impact Factor. While the numerator includes all items published in a journal, the denominator includes only those deemed citable by JCR. Items (or document types) included in the denominator are article, review, and proceedings paper. In particular, some take issue with the definition of a “citable item”. The range of JIF values and JCR’s coverage varies by discipline, which makes context such as citation density and rates crucial for interpreting the JIF.
- Journals in the Arts and Humanities Citation Index are reportedly not given a JIF due to the long half-life of citations and references in them.
- Research published in journals written in languages other than English may be at a disadvantage, as studies have suggested that these journals are not included as often in the JCR.
- Journals published in North America are reportedly disproportionately represented in the sources indexed by Journal Citation Reports.
- Review articles have a higher average citation rate than other types of articles and will influence the net value of the JIF accordingly.
Inappropriate use cases:
- As a journal level metric, the JIF should not be used as an indicator for the quality or impact of particular articles or authors. Put another way, the JIF is not statistically representative of (the citations to) individual articles and cannot summarize the quality of an author’s entire body of work.
- As a retrospective measure of past citations to a journal, the JIF is not a good predictor of whether an individual article will be highly cited. Due to the skewed distribution of citations (relatively few articles receive most citations, sometimes described as “the long tail”), the use of the mean rather than the median value of citations per article does not offer a reliable prediction for the average number of citations an article can expect to receive.
Available metric sources: Journal Citation Reports
Transparency: The formula for calculating the JIF is public, though there is some debate about the transparency of how journals are selected for inclusion (see above). For those with subscription access to the JCR, a journal’s JCR listing includes a list of the counted items in the denominator. The citation data network and the summarized values used for all metrics are available to subscribers for download.
Timeframe: Recently, the JCR has been released about 6 months after the year in question. For example, the JIF for 2016 were released in June 2017.