Impact Factors give the average number of citations received by articles in a particular journal; essentially, the average number of times that articles in a particular journal are referenced by other articles.
The Impact Factors published annually in the Journal Citation Reports® are defined as follows:
Number of citations (references) received in the Impact Factor year to articles published in the two previous years, divided by the number of articles published in these two years.
Therefore, the 2009 JCR Impact Factors (released in 2010) were calculated as follows:
No. of citations received in '09 to articles published in '08 & '07 in Journal X;
No. of articles published in '08 & '07 in Journal X;
For example, the 2009 Impact Factor for Biofouling was calculated as follows:
Citations received in 2009 to articles published in Biofouling in 2008 = 207;
Citations received in 2009 to articles published in Biofouling in 2007 = 208;
Total citations received in 2009 to articles published in 2007 and 2008 = 415;
Number of articles published in Biofouling in 2007 = 40;
Number of articles published in Biofouling in 2008 = 54;
Total number of articles published in 2007 & 2008 = 94;
2009 Impact Factor = Citations in '09 to articles published in '08 & '07;
No. of articles published in '07 & '08;
2009 Impact Factor for Biofouling = (415 ÷ 94) = 4.415.
The following are the main things that need to be considered when comparing or evaluating journal Impact Factors:
The average number of citations received by articles during the two years after publication varies considerably across different subject fields. This leads to very different ranges of Impact Factors in different subject areas. For example, the top journal in cell biology has an impact factor of more than 40. In law, however, the top journal has an Impact Factor of less than 5. This doesn't mean that cell biology journals are 'better' than law journals; it is simply a reflection of different referencing patterns and behaviour in these fields.
The Journal Citation Reports® take this variation into account by dividing the journals into subject categories. It is only within these categories that Impact Factors should be compared, and a journal's relative standing in a category is generally more important than the actual value of its impact factor. There are over 220 subject categories in the two editions of the Journal Citation Reports®.
You may see the following type of information on a Journal homepage:
2009 impact factor: 4.415
(2/88 Marine & Freshwater Biology, 19/150 Biotechnology & Applied Microbiology)
© Thomson Reuters Journal Citation Reports 2010
This means that the journal has an Impact Factor of 4.415 and this is the second highest Impact Factor out of the 88 journals listed in the "Marine & Fresh Water Biology" category and also 19th out of the 150 journals listed in the "Biotechnology & Applied Microbiology" category.
Basic v/s Applied Research
Applied journals are more likely to reference related basic research journals than other applied journals. There is no comparable flow of citations back from the basic research journals. Thus, basic research journals tend to receive more citations than related applied journals and, therefore, have higher Impact Factors. Practice-based and educational journals often have particularly low Impact Factors compared to the basic research journals in their fields. However, these journals fulfil a necessary role within their community.
Review articles are generally cited more often than primary research articles. This is because authors will often cite one review article rather than the many primary research articles it is based on. As such, review journals, or journals that publish a significant amount of review content alongside their primary content, usually have higher Impact Factors than other journals in their field.
Although the JCR® lists journals from different subject areas separately to take account of subject variation in Impact Factors, it does not list review journals separately from primary research journals. Therefore review journals are often ranked amongst the highest journals in their fields. For instance, three of the top five journals in the 2009 "Chemistry Multidisciplinary" category were review journals and the other three journals published between 6% and 60% review articles.
Journal Size & Impact Factor Variability
A journal's Impact Factor can change a great deal from year to year and the smaller the journal is, the more variable its Impact Factor is likely to be. This is because small changes in the absolute number of citations received have a much larger effect on the average number of citations when the denominator (number of articles) is small.
For example, we can consider the effect of publishing an article which receives five citations, on two theoretical journals with an Impact Factor of 1.000, one publishing 25 articles a year and the other 100 articles a year. In the small journal this will improve the Impact Factor by 0.100 (5 / (2 x 25), which is a 10% improvement. In the large journal, however, this would only lead to an improvement of 0.025 (5 / (2 x 100) which is only a 2.5% improvement.
Journals need to be selected for coverage in either the Science Citation Index-Expanded ™ or the Social Sciences Citation Index® before they will be listed in the Journal Citation Reports® and given an Impact Factor.
Thomson Reuters considers many factors when evaluating whether to cover a journal in one of its citation indexes. These include the level of citation activity to the journal from the titles that are already indexed, basic publishing standards such as getting issues published on schedule, and the international relevance of the journal. However, the subject coverage of the journal is also an important factor and journals in well-covered or low-priority subjects may struggle to get selected.
As mentioned above, it should be noted that even if a journal is selected for coverage in the Science Citation Index-Expanded ™ or Social Sciences Citation Index ®, it will be two to three years after coverage begins before the journal is listed in the Journal Citation Reports ®, unless the journal is selected for coverage in its first volume. For example, a journal selected for coverage in 2010 should be listed for the first time in the 2012 Journal Citation Reports ®.
Further information about impact can be found from several other sources including, but not limited to, those listed below, contact us on: email@example.com, if you know of others you think should be added:
Centre for Accounting, Governance and Sustainability, University of South Australia
A range of resources has been brought together to inform academic researchers on methods for selecting journals including links to various citation analysis databases (e.g. ISI, Google Scholar, Scopus, etc.). The CAGS site provides information relevant to the Australian context, especially for the government recognised Field of Research 1501 'Accounting, Auditing and Accountability'.
Economic and Social Research Council
This tool kit can help you to communicate your research and achieve maximum impact.
Impact Assessment Research Centre (IARC)
The increasing interest in evidence-based policy-making has raised new challenges and debates among impact assessment researchers and practitioners. By encouraging an integrated approach to impact assessment, the IARC at the University of Manchester seeks to strengthen the linkages between different impact assessment methodologies and practices.
National Coordinating Centre for Public Engagement
Tools and resources to help you engage with the public.