Citation data have long been used to rank journals within particular subject areas, usually based on the Impact Factor (IF). The IF is a numerical ratio of the total number of citations a journal receives in Web of Science Source Journals in one year to the total number of "citable" articles it published in the previous two years. It is useful to see how journals perform in relation to others in the same subject area. It is not useful in comparing journals across subject areas, and the number taken out of this context is essentially meaningless.
For example, let's say that Journal A has an impact factor of 4.327, and Journal B has an impact factor of 1.045. Is Journal A "better" than Journal B? You could conceivably make that argument, if you first accept the notion that citedness is a measure of quality, AND if journals A and B are both in the same field of study. But if A is in Biochemistry, and B is in Clinical Pharmacy, no such judgment can be made, as citation behavior varies considerably from field to field. Similarly, small differences in the numeric IF (<1.0) are not significant: for example, two journals with respective IFs of 2.235 and 1.962 are not significantly different.
Impact factor can also vary based on the number and types of articles a journal publishes. Review articles tend to be more heavily cited than research papers or short communications, so journals and annuals that publish many reviews will often have higher impact factors. Journals that publish only a few articles in a given year may also have disproportionately high impact factors. Similarly, one very highly cited paper in one year can temporarily skew a journal's impact factor.
Impact factors for journals covered by Web of Science are published annually in InCites Journal Citation Reports. All WOS Source Journals are ranked within one or more relevant subject categories, such as CHEMISTRY, ORGANIC or SPECTROSCOPY. You can also compile customized lists. JCR also contains data on historical trends, immediacy index, cited half-life, etc.
While Impact Factors are useful within certain limits, they are subject to manipulation. Critics point to a lack of transparency and reproducibility as an ongoing problem. Nor does a journal's IF relate in any way to the impact or quality of an individual author or paper. For these reasons, the use of journal impact factors in personnel and funding decisions is strongly discouraged.
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