HealthMay 12, 2020

What is the impact factor of a journal?

By: Tayler Sindhu, MD

Impact factors are a controversial yet widely used framework to measure a journal’s influence. Here’s a closer look at what impact factors are, how to find them, why they matter and how to use them.

What is the impact factor of a journal?

At its core, an impact factor is a measure of citation frequency, which offers a quantitative metric for evaluating journals and assessing their relative influence. Impact factors are assigned to journals across a wide variety of academic disciplines.

Where do you find a journal’s impact factor?

The Web of Science Group’s Journal Citation Reports (JCR), a widely utilized subscription-based institutional resource, has provided impact factors for select journals since 1975. Publishers, journals and search engines also typically provide impact factors; however, they are not guaranteed to be accurate or up to date, as an article in the Journal of Bone and Joint Surgery emphasizes.

Check out Wolters Kluwer Health journals here.

How is a journal impact factor calculated?

JCR calculates the impact factor of a subset of journals in the Web of Science Core Collection each year as follows: The number of citations in a given year to articles that a journal published in the prior two years, divided by the number of citable articles that the journal published in that same time frame.

Citable articles include reviews, articles, and proceedings papers, but excludes items such as editorials and letters. For example, in the 2019 Journal Citation Report, the impact factor for the New England Journal of Medicine (NEJM) was 70.670. This is the number of 2018 citations to NEJM articles published from 2016 to 2017 (46,289), divided by the total number of citable articles NEJM published during those years (655).

Notably, this was the third highest impact factor across all categories in the 2019 JCR. CA: A Cancer Journal for Clinicians achieved the highest impact factor, 223.679. As different fields may have different publication rates and citation patterns, it’s wise to avoid directly comparing journals in different fields according to their impact factors.

Why does impact factor matter?

The emphasis on journal impact factor is controversial within the scientific community. As discussed in Nature, this controversy stems from the concern that impact factor may not adequately capture journal quality or influence and could contribute to an “unhealthy research culture” aimed at improving impact factors.

Despite this controversy, research institutions use impact factors in evaluations and promotion decisions, and impact factor is widely used as a surrogate for journal importance.

How do you use a journal’s impact factor?

Impact factor is just one metric to assess the quality and relative importance of a journal. Obtaining journal impact factors can be a good preliminary step to take when deciding where to submit an article. The JCR categorizes journals according to the majority of medical specialties, allowing users to view journals in their field according to their impact factors.

Impact factors can also help you avoid journals that engage in “predatory publishing,” as journals undergo a vetting process before qualifying to receive an impact factor. As detailed in a “JCR Editorial Expression of Concern,” the Web of Science Group investigates and suppresses journals from calculations who exhibit “problematic patterns of citations.”

What are the limitations of journal impact factors?

While useful, journal impact factors must be interpreted carefully. As explained in the Journal of the Medical Library Association, impact factors do not necessarily speak to the caliber of a journal’s peer review process or its articles. Additionally, the process may be skewed toward more established journals, as a journal must be indexed for several years to receive an impact factor.

The ideological founder of the impact factor, Dr. Eugene Garfield, notes that factors such as the types of articles a journal publishes may influence its impact factor. For example, review articles are often cited more frequently than other article types. Consequently, journals that publish a high proportion of review articles may be cited more often, resulting in a higher impact factor.

To mitigate this effect, JCR offers transparency into the factors comprising a journal’s impact factor in “Journal Profile Pages,” including the distribution of the types of cited articles that a journal publishes.

While they have limitations, impact factors are an important objective metric of journal influence that can help you decide where to submit articles.

Tayler Sindhu, MD
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