Published 1/1/2012
David J. Tennent, BS; Chad A. Krueger, MD; Daniel J. Stinner, MD

What Is the “Impact Factor”?

The decision of where to publish research results is one that every investigator takes into account when preparing a manuscript for publication. The topic of the investigation, the quality of the research, and the opportunity to publish in the most highly regarded journal all play a part in the researcher’s final decision.

But how does a journal get a reputation as being “better” than other journals? Many researchers generally accept that a journal’s reputation is based on the following factors: history of publication, breadth of readership, quality of individually published articles, selectivity in deciding which articles to publish, and impact on the practice of orthopedics.

History, readership, article quality, and selectivity may be independent of the clinical relevance of any particular journal, and are fairly easy to measure. The impact that a particular journal has on the practice of care, however, is less tangible and more difficult to quantify.

The most popular attempt to quantify the impact, or importance, of a particular journal was described by Eugene Garfield, PhD, in 1955 when he developed the impact factor. Dr. Garfield described the impact factor as “based on two elements: the numerator, which is the number of citations in the current year to any items published in the journal in the previous 2 years; and the denominator, the number of substantive articles (source items) published in the same 2 years.”

This quantification method was primarily designed to assist the Science Citation Index in selecting which journals to include in its core journal index, based on the relative amount of citations originating from a particular journal. Its use has been further extrapolated by libraries and journal indexes to determine subscription policies and by authors to measure which journal will enable their article to have the greatest clinical influence.

The online version of this article (available at www.aaos.org) lists the top orthopaedic journals, based on their impact factor, as of November 2011. This does not mean that the articles published in more highly ranked journals are any better than those published in lower ranked journals because the impact factor is simply an average of all citations from a particular journal. It also does not imply that the quality of one journal is any better than another or that the readership of one journal is greater than another. However, it does say that articles in the higher ranked journals are cited more frequently than those in other journals.

Factors influencing the impact factor
In general, review articles and technical reports are more frequently cited in the general medical literature than peer-reviewed original research. Consequently, journals that publish review articles rather than original research tend to have higher impact factors.

A 2007 review of articles published in The Journal of Bone and Joint Surgery–American (JBJS–A) during 2000 found that randomized control trials, meta-analyses, and basic science research were most likely to be cited in both the orthopaedic and non-orthopaedic literature. Accordingly, the publication of these types of articles in a journal could theoretically be more beneficial as it may result in a greater increase in the impact factor of the journal than publication of retrospective studies and case reports.

Self-citation also greatly influences the impact factor of orthopaedic literature. A 2006 study found that, over a period of 2 years, up to 40 percent of an orthopaedic journal’s citations originated from within the same journal. Furthermore, subspecialty journals with little overlap into general orthopaedics tended to have higher self-citation rates due to the highly focused nature of their included studies. Whether self-citation represents a journal’s propensity to accept articles similar to those previously published or an author’s bias to cite articles from journals in which the author hopes to be published is unclear. What is clear is that self-citation disproportionately increases a journal’s impact factor.

Criticism of the impact factor
Many criticisms of the impact factor exist due to its susceptibility to be inappropriately influenced by self-citation, article selection, and the inclusion criterion of source items. In recent years, this has led to the development of more complex algorithms that attempt to more objectively determine the relative importance of different journals. Several of these methods, such as the Eigenfactor and SCImago, weight journals differently based on their number of citations in other journals. The algorithms that these methods use closely resemble the way that the Google® search engine prioritizes Web search results and can also be used to make determinations of individual author contributions.

As orthopaedic research continues to increase, authors will continue to seek publication in journals that are perceived to have the greatest clinical impact. Physicians, who have a finite amount of time, will continue to prioritize what journals they read to maximize their learning. Although not perfect, the impact factor is a method that residents and practicing surgeons can use to select journals that theoretically have a higher concentration of influential articles. Selecting only journals with high impact factors, however, may mean that a reader will miss important articles that could greatly influence his or her practice.

David J. Tennent, BS, is a 4th year medical student at The University of Texas Medical School at Houston. Chad A. Krueger, MD, is the resident member of the AAOS Communications Cabinet. Daniel J. Stinner, MD, is a chief resident at the San Antonio Military Medical Center, San Antonio, TX.