Journal impact factor reflects the frequency an average article from a journal is cited in a given year. Conceived by Eugene Garfield in 1955, the journal impact factor can be used to compare a journal's relative importance to others in the same discipline. It minimizes the advantages of larger, older and more frequently published journals.
Journal impact factor is defined as the average number of times articles from a journal published over a two year period have been cited the following year. The impact factor is calculated by dividing the number of citations in a selected year by the total number of research and review articles published in the two preceding years. So, for example, an impact factor of 1.0 signifies that articles published in the last year or two have been cited once on average.
Journal impact factors are obtained through the Web of Knowledge database using the Science Edition of Journal Citation Reports® which collects citation data from more than 7,300 worldwide science and technology journals.
A 5-year journal impact factor is also available through Journal Citation Reports®. The 5-year journal impact factor is the average number of times articles from a journal published over a five year time span have been cited in a succeeding year. It is calculated by dividing the number of citations in a given year by the total number of articles published over the previous five years.
Note: The 5-year impact factor is available from 2007+. Also, journal impact factors may be obtained without self-citations by going into the detailed view in Journal Citation Reports in Web of Science.
- Citations are not weighted by the prestige of the citing journals.
- Frequency of publishing (weekly, monthly, quarterly, biannually) has some effect.
- Differences in citation patterns among disciplines are not taken into account, therefore, journals can not be compared across different fields.
- A high citation rate of an article does not necessarily equate to high quality as the context of citations is not considered.
- Articles may reach their maximum impact beyond the 2 year time frame.
- Citations made in journals which are not indexed by Web of Science are excluded.
- Books, book chapters, and conference proceedings are not considered.
- Non-English articles receive few citations due to the fact that the majority of the scientifc community can not read them.
- There may be bias due to the inclusion of self-citations, however, journal impact factors without self-citations may be obtained from the detailed view of Journal Citation Reports in Web of Science
- Minor errors in references may lead to slightly skewed measures.
- In the area of medicine and healthcare, research-oriented articles are more heavily cited, whereas clinical practice articles are typically acted upon rather than cited.
- Not all journals have an impact factor; to compare journals using other metrics, try the Compare Journals tool in Scopus