Journal Impact Factor
The Journal Impact Factor is used for the evaluation of journals. It expresses the number of citations per article in a journal, or more exactly, the number of received citations per article from the current year to articles from the two precedent years.
The Journal Impact Factor is found in the Thomson Reuter's Journal Citation Reports. Not all journals are covered and hence, not all journals have a Journal Impact Factor. The citations are calculated from the Web of Science.
One should keep in mind that the distribution of citations between publications is highly skewed, i.e. rather few publications get a high share of the total number of citations and a high number of publications get non or just a few. This means that the Journal Impact Factor is no good predictor of the citation success of a single article.
- Indicates the impact of a journal objectively. There was no such statistical indicator before the Journal Impact Factor.
- Relatively easy to understand since the index explicitly reflects the number of citations.
The Journal Impact Factor is a poor predictor of the impact of a single article since the distribution of citations between articles is highly skewed. Criticism has therefore been raised against using Journal Impact Factor at the article level or for the evaluation of researchers or research groups.
No normalization for research field or for document type. It is therefore easier for journals within some fields to get a high Journal Impact Factor. There are for example a low number of citations per paper within Arts and humanities, Computer Science and Mathematics and high numbers of citations per paper within Cell biology, Chemistry and Physics. Hence, the Journal Impact Factor can not be compared between journals within different research fields.
- Review journals and journals with editorial material and other such materials are favored (e.g. Nature and Science). This is because citations from all kinds of documents are counted but only articles and reviews count as papers in the calculation. Another reason is that review articles get more citations and the Journal Impact Factor does not normalize for document type.
Althouse, B. M., West, J. D., Bergstrom, C. T., & Bergstrom, T. (2009). Differences in impact factor across fields and over time. Journal of the American Society for Information Science and Technology, 60(1), 27–34. doi:10.1002/asi.20936
The San Francisco Declaration on Research Assessment (DORA) - URL: