Impact factor of scientific journals
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Definition and Calculation of Journal Impact Factor
The impact factor (IF) is a widely used metric to assess the influence of scientific journals. It is calculated as the average number of citations received in a given year by articles published in the journal during the previous two years. Specifically, IF = (Citations in Year X to articles published in Years X-1 and X-2) / (Number of citable items published in Years X-1 and X-2) 1789. This metric was originally developed to help librarians decide which journals to subscribe to and has since become a standard tool for comparing journals 128.
Use and Misuse of Impact Factor in Scientific Evaluation
Although the impact factor was designed to compare journals, it is often misapplied to evaluate the quality of individual articles, researchers, and even institutions 1256. This practice is problematic because the impact factor does not accurately reflect the quality or citation impact of individual papers, and there is often a poor correlation between a journal’s IF and the actual citations of its articles 256. Studies have shown that highly cited articles can appear in lower-IF journals, while some articles in high-IF journals may receive few citations 2.
Limitations and Criticisms of the Impact Factor
The impact factor has several well-documented limitations. It is biased toward journals from large, wealthy publishers, as these journals have greater visibility and thus more opportunities for citation, disadvantaging journals from less economically developed regions 3. Editorial practices can also artificially inflate IFs, such as encouraging self-citation or publishing more review articles, which tend to be cited more frequently 1. The definition of "citable items" is inconsistent, and some document types (like letters and editorials) are often cited but not counted as citable items, leading to inaccuracies in the metric 19.
Impact Factor Distribution and Field Differences
The distribution of impact factors across journals is stable year-to-year and follows an exponential pattern with a power-law tail, meaning a small number of journals have very high IFs while most have much lower values 4. Additionally, impact factors vary significantly between scientific fields, making cross-field comparisons problematic. Methods such as the standard impact factor for specific fields have been proposed to address this issue 10.
Alternatives and Calls for Reform
Due to its limitations, there is growing support for alternative metrics, such as the Eigenfactor, which aim to provide a more balanced assessment of journal influence 3. Major scientific organizations and funding agencies have called for moving away from using IFs to evaluate individual researchers or research groups, as highlighted by the San Francisco Declaration on Research Assessment (DORA) 12.
Conclusion
The impact factor remains a prominent metric for evaluating scientific journals, but its misuse and inherent limitations have led to widespread criticism. While it offers a simple way to compare journals, it should not be used to assess individual articles or researchers. The scientific community is increasingly advocating for more nuanced and fair approaches to research evaluation, including the adoption of alternative metrics and field-specific assessments 123.
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