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. The impact factor's only valid purpose is to compare a journal's citation rate 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 make that argument, if 1) you first accept the notion that citedness is a proxy for quality, AND 2) 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 IF value (< 1) are not significant: for example, two journals with respective IFs of 2.235 and 1.962 are not significantly different, because they are both around 2. Because IFs are listed with three decimal places, they are often misunderstood as being more precise than they actually are.
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 Clarivate's 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.
Impact Factors are only available for journals covered in Science Citation Index-Expanded (SCIE) and Social Science Citation Index (SSCI) segments of Web of Science, and only for titles that have at least three full years of citation data available. Impact Factors are most established in basic science and clinical fields. They are somewhat less useful in technology and social science fields, where communication is less journal-centric. Impact Factors are not calculated for Arts & Humanities Citation Index (AHCI) journals.
While Impact Factors are informative within certain limits, their use and value have been the subject of much debate. Critics point to a lack of transparency and reproducibility, and their proprietary nature. They are also susceptible to editorial manipulation. There is no definition of what a "good" impact factor is, since it is only a relative measure. Publishers often use them out of context for marketing purposes.
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