The Journal Impact Factor is a ratio devised as a measurement of the average citedness of a journal. It is calculated by dividing the number of citations to a particular publication in a certain year by the number of citable articles published in the journal in the previous two years. Other related measurements may also be calculated, such as the Five-year impact factor (which looks at the previous 5 years), or the Immediacy index (which divides the number of citations in a given year by the number of citable articles in that same year).
Journal Impact Factors are not a measure of the citedness of any particular article in a journal, but rather of the journal as a whole. They also can be influenced by other considerations. For example, a journal that publishes mainly review articles may have a higher impact factor as these are more often cited.
Journal Impact factors are published in the Journal Citation Reports database, which covers both the sciences and social sciences, but not the humanities (though History is included in social sciences).
You can search for a specific journal to determine it's impact factor in any associated categories, or browse a list of all journals in a particular subject category, which can be ranked by a variety of impact factor metrics. This way of viewing impact factors is recommended, as the impact factor can vary widely from discipline to discipline depending on citation practices in that field.
To look up a single journal, you can type the journal name or abbreviation to bring up a report on that journal. The report will include a graph of its current and recent impact factor compared to other journals in its same categories, other journal impact metrics, and links to category data.
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To browse all of the journals in a particular subject category, choose 'Browse by Journal' and then Select Categories at the left the click Submit. The resulting list will be ranked by Journal Impact Factor within the category. Eigenfactor will also be displayed, and other metrics are available if you log in to a (free) account. Click on any column heading to sort by the column.
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More information about the Journal Impact Factor can be found in:
Garfield, Eugene. "History and Meaning of the Impact Factor." JAMA 295, no. 1(2006): 90-93.
The journal impact factor was designed for only one purpose – to compare the citation impact of one journal with other journals. While originally created to measure of the frequency with which the “average article” in a journal has been cited in a particular year, it also increasingly used to evaluate a journal’s relative importance with others in the same field. It has been increasingly evident that the best journals within each specialty are those “in which the it is most difficult to have an article accepted, and these are the journals that have a high impact factor.”
The impact factor is not only increasingly used to measure the quality of specific journals (as the best or most prestigious). Time, budget, and administrative pressures tempt many to use this readily available citation metric not only to support, but to substitute for informed peer judgment in research evaluation. This tool is also being used erroneously as the sole or major factor in assessing the “quality of scientists, institutions, and even scientific research.”
Common misuses of the impact factor as a sole criteria
An impact factor indicates to some extent the quality of a journal as a whole. However, the impact factor alone does not indicate the quality of individual articles within a journal, the overall quality of the research performed authors publishing within journals with impact factors, or the prestige of associated academic departments, research programs, or institutions.
The Scopus Database (Elsevier) is another well-known source of citation data. They have developed three metrics to measure journal impact derived from their data. The University of Toledo does not have access to the entire Scopus database, but impact measures for journals are freely available:
Eigenfactors are a measure not only of cited by number of citations to a journal, but also where those citations come from. Eigenfactor and Normalized Eigenfactor scores are also reported in Journal Citation Reports.