Journal Impact Factors
Impact Factor is purported to be a measure of importance of scientific journals with the higher the factor, the more important the journal. It is calculated each year by the Institute for Scientific Information (ISI) for those journals which it tracks (which are numerous). Impact factors are published in the Journal Citation Report. Impact Factors have a growing, but controversial, influence on the way published scientific research is perceived and evaluated.
Calculation
The Impact Factor is usually calculated based on a three year period. For example, the 2004 Impact factor for a journal would be calculated as follows:
A = Number of times articles published in 2001-2 were cited in tracked journals during 2003
B = Number of articles published in 2001-2
2003 Impact Factor = A/B
There are some alterations: ISI exclude certain article types (e.g. news items, correspondence, errata) from the denominator. Also, for new journals, ISI will sometimes calculate an Impact Factor with two years of data.
Pros and Cons
It is sometimes useful to be able to compare different journals when you are looking for the best place to send a paper. The impact factor is fairly universal (more than 8400 journals worldwide), objective and freely available. More institutions of higher learning, especially those in Europe, are using the impact factor as a metric during reviews for academic appointments and promotions.
Impact factors are not, however, perfect. First, the number of citations is not a complete measure of the quality or importance of an article. Second, in a journal which has long publication times, it might be impossible to cite articles new enough to fall inside the three year window. For some journals, the time between submission and publication can be over two years, which leaves less then a year for citation. The nature of results in different research areas yield different amounts of publications and at different pace, which has an effect on impact factors. Generally, for example, medical journals have higher impact factors than engineering journals.
Where do I find current impact factors?
There are specific web sites that list yearly impact factors by journal title. Link to a website where impact factors can be found is given below:
The ISI website
Click below to watch the video on using ISI Web of Science for impact factors:
The Journal Database
Thomson ISI tracks a set of more than 5,000 internationally recognized journals that it has determined to be the most influential in the world. S&E articles in these journals build primarily on one another to form the scientific literature. These journals contain approximately 15 million citations, about 85% of which are to articles in journals in the Thomson ISI database. Coverage extends to electronic journals, including print journals with electronic versions and electronic only-journals. Journals of regional or local importance may not be covered, which may be especially salient for research in engineering/technology, psychology, the social sciences, the health sciences, and the professional fields, as well as for nations with a small or applied science base. Thomson ISI covers non-English language journals, but only those that provide their article abstracts in English, which limits coverage of non-English language journals. Relative to other bibliometric databases, Thomson ISI indexes a wider range of S&E fields and contain more complete data on the institutional affiliations of article's authors. For particular fields, however, other databases provide more complete coverage.
Footnote: Thomson ISI changed its name to Thomson Scientific in June 2006.
Expanding and Fixed Journal Sets
A second issue is how to deal with changes in the set of journals tracked by Thomson ISI. Over time, many new journals emerge and attain influence, while a few older journals decline or stop publication. Because the global S&E research enterprise is growing, the net direction of change is more articles and more journals in the Thomson ISI database. The database grew from 4,460 journals in 1988 to 5,262 in 2001, and many of the journals indexed published more articles per issue and more issues per year toward the end of the period than they did in previous years.
At any given time, the expanding set of journals tracked by Thomson ISI is the most suitable indicator of the mix of journals and articles. Patterns of authorship and citation in this set reflect the fields, nation, and institutions in which high-quality research is being produced. However, an expanding set of journals poses problems for trend analyses. Changes in the expanding set over time can result not only from changes in how and where scientists and engineers perform research, but from changes in the journals Thomson ISI chooses to include or the depth of its coverage in different fields of languages.
Footnote: Among the journals included in the Thomson ISI database since 1985, the average annual number of articles per journal rose from 102 in 1986 to 142 in 1999.
The text in shaded area is taken from Hill et al., 2007
A New Journal Ranking Service
A new and free website is recently launched providing a journal ranking service to its visitors. The website, www.journal-ranking.com, introduces two new indicators relevant to journal quality based on journal influence index and the paper influence index.
The service allows registered members to validate various scenarios and parameters to rank journals. It also facilitates a forum platform to enable feedback from the audience.
The guests can browse the ranking results generated under 27 ranking scenarios whereas the members can access their specific ranking domains and create scenarios.
A recent study of NSF emphasizes the importance of impact factors and citations (The text in the link is taken from Hill et al., 2007)
Some articles on impact factors
Kuo, W., Rupe, J., "R-Impact: Reliability-Based Citation Impact Factor," IEEE Transactions on Reliability, September 2007, Vol. 56, No. 3, 366-367.
Amin, M., Mabe, M., "Impact Factors: Use and abuse," Perspectives in Publishing, October 2000, No. 1: 1-6,
URL:http://www.ntu.edu.sg/home/mwtang/ifuse.pdf
Lundberg, G., "The 'omnipotent' Science Citation Index Impact Factor," The Medical Journal of Australia, 2003, 178(6): 253-254
URL:http://www.mja.com.au/public/issues/178_06_170303/editorials_170303-2.html
MacRoberts, M., "Rejoinder: Validation of Citation Analysis," Journal of the American Society for Information Science, 1997, 48(10): 963
"Not-so-deep impact," Nature 435, 1003-1004, 23 June 2005
Saha, S., Saint S., and Christakis, D.A., "Impact factor: a valid measure of journal quality?," Journal of the Medical Library Association, January 2003, 91 (1): 42-46,
URL:http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=141186
Sodhi, M. S., "Journal Rankings: U.K. Perspective," OR/MS Today, 2005, Vol. 32, no. 2, p. 14.
Trayhurn, P., "Citations and 'impact factor' - the Holy Grail," British Journal of Nutrition, July 2002, 88(1): 1-2
URL:http://www.nutritionsociety.org.uk/bjn/088/bjn0880001.htm