Journal Impact Factor
Can apply to: Journals selected by Clarivate Analytics staff for indexing in Web of Science (Science Citation Index Expanded and/or Social Sciences Citation Index)
Metric definition: The Journal Impact Factor (JIF) is a ratio, scaling the number of citations received by a journal by volume of recently published scholarly materials in that journal
Metric calculation: From Clarivate Analytics: “The JIF is defined as citations to the journal in the JCR year to items published in the previous two years, divided by the total number of scholarly items, also known as citable items [articles and reviews], published in the journal in the previous two years.”
Data sources: Web of Science
Appropriate use cases: The JIF can be useful in comparing the relative influence of journals as measured by citations. Used appropriately and in conjunction with other metrics, the JIF can be useful in collection development decisions made by librarians.
Limitations: 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 JCR impact factor is a ratio between citations and recent citable items published.” The widespread use of the JIF, particularly applied to articles and authors, has resulted in its corruption through gaming by journal editors, perverting incentives, and misuse as a measure of the quality or impact of individual scholars or articles. Here are the most commonly noted limitations of the JIF:
- There is a lack of transparency about the database of journal articles on which the JIF is calculated. More specifically, Thomson Reuters does not provide clear, concrete guidelines for the journals and article types selected for inclusion.
- In some disciplines, 2 years is not enough time for articles to accrue citations.
- Coverage varies by discipline, particularly for the arts and humanities, which makes context crucial for interpreting the JIF.
- In many fields, the correlation between JIF and citation rates for a particular article is weak.
- There is a strong English language bias in the journals covered.
- There is a strong American bias in the journals covered.
- The JIF is affected by many cultural differences across disciplines (e.g., frequency of publication, typical number of citations per article, average length of article, etc.) that prevents it from being useful in comparing metrics across disciplines.
- Interdisciplinary journals are not well represented in the JCR database.
- Editorial policies can affect the JIF.
Inappropriate use cases: Help documentation provided by Thomson Reuters states “The Journal Impact Factor is a publication-level metric. It does not apply to individual papers or subgroups of papers that appeared in the publication. Additionally, it does not apply to authors of papers, research groups, institutions, or universities.”
- As a journal level metric, the JIF should not be used as an indicator for the quality or impact of particular articles or authors.
- The JIF should be presented with appropriate context.
- The JIF is not a good predictor of whether an individual article will be highly cited.
- The JIF should not be used as an estimate of attention, discussion, or scholarly impact. Citations do not reflect all scholarly uses of the literature.
- The JIF is not a good statistical proxy for the citation rate for individual journal articles. Due to the skewed distribution of citations (relatively few articles receive most citations, otherwise known as the long tail), the use of the mean rather than the median value of citations per article is misleading.
- The type of article can have significant impact on the number of resulting citations. For example, review articles are more frequently cited than other types of articles.
Available metric sources: Journal Citation Reports
Transparency: The process of selecting journals and article types for inclusion in the Journal Citation Reports database is not clearly documented, though the formula for calculating the JIF is public.
Timeframe: The JCR is typically released two years after the year in question. For example, the JIF for 2015 were released in 2017.