How is a Journal’s Impact Factor Calculated, and Why Does it Matter?
Although there are a variety of estimates of a journal’s influence, the impact factor commonly remains the most referenced. First introduced in the 1960s to assist librarians, it represented an important improvement over a simple count of citations that could make the value artificially high if a journal published an unusually large number of articles per year.
The impact factor calculation considers the number of citations articles garnered and the number of articles published over two years. So, the number reflects the average number of citations for an article in the journal over a specific (and recent) period of time. Only scholarly articles are included (original research, reviews).
At Acumen, we are often asked to advise on impact factor cutoffs for journal selection. In other words, what impact factor is considered “good?” However, a strict cutoff is rarely appropriate. Understanding how the impact factor is calculated and exceptions to its utility is important for selecting the right journal for your manuscript. Below are factors that influence the impact factor and should be considered when selecting a journal for your manuscript:
Citation data: The impact factor is based on the number of times articles published in the journal are cited by other researchers in their own publications. The number of citations a journal receives is a measure of its influence and the importance of its research. A strength of the impact factor is that it considers the number of articles published and the number of references to the journal. However, the types of articles published by a journal can influence citation rate like the frequency of “special issues” that publish on popular topics that are referenced more frequently.
Time frame: The impact factor is calculated over a two-year period, with the number of citations received in the current year being divided by the total number of articles published in the previous two years. A newer journal could have a lower impact factor for several years before it gains recognition.
Journal size: The total number of articles published by the journal in the two-year period is also a factor in determining the impact factor. Journals that publish a higher number of articles are expected to receive more citations, which may influence their impact factor.
Field of research: The impact factor varies between different fields of research, reflecting the differences in citation practices and the relative importance of different research areas. A journal’s impact factor could appear artificially low if it publishes articles in a narrow field of interest, like a rare disease, or artificially high if it published on a topic of high interest like oncology or molecular biology or multidisciplinary journals. So, it is important to consider how broad or popular the publishing interests of the journal area.
Language: The impact factor is also influenced by the language in which the journal publishes its articles. Journals that publish in languages other than English may receive fewer citations and therefore have a lower impact factor.
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Pam Harvey, Lead Medical Publications Writer
