How Journal Impact Factors Are Calculated and What They Measure
A journal impact factor is the average number of citations in a given year to items the journal published in the previous two years. To calculate it, divide those citations by the number of items published during the same two-year period. The figure describes citation activity around a journal’s recent content, not the quality of any particular paper.
Eugene Garfield devised the measure at the Institute for Scientific Information. Clarivate now publishes journal impact factors in Journal Citation Reports Wikipedia — Impact factor.
The formula and an example
For a journal in year Y:
Impact factor = citations in Y to items published in Y−2 and Y−1 ÷ items published in Y−2 and Y−1
The numerator counts citations from indexed publications to the journal’s items in the two-year window. The denominator counts items published in that window. The result is an average, not a total citation count.
For example, the cited calculation for Nature in 2017 uses 74,090 citations to items published in 2015 and 2016, divided by 1,782 items from those years. The result is approximately 41.6 Wikipedia — Impact factor.
The calculation does not account for differences between fields or for the proportions of research articles, reviews and other content. Which items enter the denominator depends on how the journal’s content is classified for the calculation.
What the number measures
An impact factor reports how often a journal’s recent content was cited during a particular year, averaged across the items counted in the denominator. It does not show how often each paper was cited. A few highly cited items can make a substantial contribution to the average, while many other items may receive few or no citations.
The metric does not assess a paper’s methods, accuracy or practical value. A citation can refer to work that is being questioned or corrected, or to work that a later article builds on. The calculation counts citations but does not assess their meaning.
| Feature | What it means |
|---|---|
| Two-year window | Citations to older work do not enter that year’s calculation |
| Journal-level average | The figure does not describe every article in the journal |
| Denominator based on classified items | The types of content counted can affect the result |
| Citations from indexed publications | The calculation does not judge why a work was cited |
Limits to keep in mind
Citation patterns differ across disciplines. In some fields, relevant work may take longer to attract citations than in others. A short citation window can make cross-field comparisons misleading. Use subject categories in Journal Citation Reports to compare journals in a similar area. A figure from one field may not be directly comparable to one from another Clarivate Journal Citation Reports.
Content mix matters, too. Review articles often attract citations, so a journal that publishes many reviews may have a different citation profile from one focused mainly on original research. The impact factor does not separate those content types in the final average.
Self-citations from a journal to its own recent content count toward the total. Their presence alone does not show improper behavior, but it is one reason to consider more than a single figure when assessing a journal.
How to use an impact factor
Use the figure as a limited point of comparison, preferably among journals in the same subject area. It can give a rough indication of citation activity around a journal’s recent content. It cannot establish the quality of an individual article or measure a researcher’s contribution.
For a specific paper, examine its methods, evidence and relevance to the question at hand. Article-level citations can provide context, but they also need interpretation. Scopus journal metrics offer other journal-level indicators for additional context Scopus Journal Metrics.
Garfield cautioned against using the impact factor to evaluate individual papers or researchers Eugene Garfield, “The History and Meaning of the Journal Impact Factor”. A journal average cannot tell you how strong every article in that journal is.
Key Takeaways
- The impact factor divides citations in a given year to a journal’s items from the previous two years by the number of items published in those two years.
- The example for Nature in 2017 gives approximately 41.6, based on 74,090 citations and 1,782 items.
- It measures journal-level citation activity over a short window, not the quality of an article or researcher.
- Field differences, content mix and self-citations can affect how the number should be interpreted.
- Compare journals within a subject area, and assess individual papers on their own evidence.
