What are impact factors?
An impact factor measures how often the "average article" from a particular journal has been cited in the past two years. The higher the number, the more other people have cited articles from the jourrnal. Many people believe that the higher the impact factor for a journal is, the more important the journal is as a scholarly and influential journal.
For example, if a journal has an impact factor of 3 in 2008, then its papers published in 2006 and 2007 received 3 citations each on average in 2008. The 2008 impact factor of a journal would be calculated as follows:
- A = the number of times that articles published in that journal in 2006 and 2007 were cited by articles in other journals during 2008.
- B = the number of articles in that journal during 2006 and 2007
- 2008 impact factor = A/B.
Some drawbacks to impact factors:
- Citation rates vary widely from field to field and shouldn't be taken at face value, but considered relative to the field of research
- Citation rate may be based on a few prolific authors citing each other, including self citations
- Citation searching works better for journal articles than books
- Coverage of your particular field in the citation database may be weak
- Cross-disciplinary research may produce fewer citations
- Distribution of the citations over time might be more indicative of their importance than an initial high citation count shortly after publication
- Impact factors are a measure the journal as a whole, not individual papers or researchers
- Impact factors may play a too important role in hiring and tenure decisions without considering other measures
- Some journals also cite articles in editorials, reviews, news and other non-research articles to increase the number of cites
- The research may too recent and not widely known, like emerging fields
- The quality of the journal producing the citation
- There is a growing tendency of some researchers to go after topics likely to get into high-impact journals, which jeopardizes creativity, can skew the course or even slow the pace of science
- There is no guarantee that every paper which ought to be cited will be cited. An un-cited author may be ahead of his peers. Mendel and his genetics work went unappreciated for years