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Medical Care blog

Comments from the scientific community regarding Medical Care.

Thursday, July 23, 2015

What's in an impact factor?

by Jessica A. Williams, PhD, MA

Medical Care’s impact factor for 2014 was 3.232! This is great news and is a testament to the journal’s quality and relevance to researchers and practitioners in the field. Our 5-year impact factor is even higher: 3.687.


What is an impact factor? It is a measure of the average citation frequency for a particular journal over a particular period of time.1

An interesting paper on the impact factor and its use (by Eugene Garfield, the originator of the concept) is available here.

Medical Care’s 2014 impact factor is the number of all citations in all journals in 2014 to the Medical Care articles published in 2012 and 2013 divided by the number of items published in 2012 and 2013.

Every field has its own list of top journals--the ones that everyone reads. One way to tell which journals are on that list is by their impact factors.  

There are a few ways to calculate impact factors, such as by removing self-citations,* but the impact factor we cite here is the Thomson Reuters version. Despite its controversies,2, 3 their standardized calculation does give the impact factor an advantage over other methods used to measure the importance of journals, particularly within a field.

The number of journals in the rankings have increased over time. For example, in 2010, there were 72 journals in the Health Care Sciences & Services category, while in 2014 there were 89 journals (roughly a 24% increase). With such an expanding number of journals, librarians and others might have greater need for tools such as impact factors to make decisions in an environment of scarce resources.

Researchers often use impact factors to make decisions about where to submit articles. However, impact factors are manipulable (such as by the timing of issues and special issues), and they vary by field as well as by article type. Different fields have different citation practices. Additionally, review times may affect impact factors: if reviews take a long time, the articles cited in papers may be too old to count in the journal’s impact factor by the time they are published.

As you might guess, review articles and review journals have higher impact factors than other types of articles or journals.1 Other data, such as the number of review studies per journals and the average number of references cited per article (reviews tend to have many citations) can provide further information about journals with very high impact factors.1

There were several highly cited articles in Medical Care covering a variety of topics during 2012-13. Some of the most highly cited papers from 2012 were “Effectiveness-implementation Hybrid Designs Combining Elements of Clinical Effectiveness and Implementation Research to Enhance Public Health Impact” by Curran and coauthors; “Prevalence and Costs of Co-occurring Traumatic Brain Injury With and Without Psychiatric Disturbance and Pain Among Afghanistan and Iraq War Veteran VA Users” by Taylor and coauthors; and “Systematic Review of Comorbidity Indices for Administrative Data” by Sharabiani, Aylin, and Bottle.

Among articles published in 2013, “Lower Mortality Magnet Hospitals” by McHugh and coauthors; and “Hospital Nursing and 30-Day Readmissions Among Medicare Patients With Heart Failure, Acute Myocardial Infarction, and Pneumonia” by McHugh and Ma were highly cited in 2014.

Check out these articles - they cover important topics in Health Services research. Maybe your article will be on next year’s list!

Jessica A. Williams, PhD, MA is a Robert Wood Johnson Foundation Health & Society Scholar at the Harvard Center for Population and Development Studies at the Harvard School of Public Health in Cambridge, MA.

*Worth noting: in Medical Care, self-citations make up only 2-3% of citations, whereas the average is around 13% of a journal's total citations.1  

  1. Thomson Reuters. “The Thomson Reuters Impact Factor.” Available at: Accessed 7/17/2015.

  2. Seglen PO. "Citations and journal impact factors: questionable indicators of research quality." Allergy. 1997 Nov;52(11):1050-6.

  3. Hoeffel C."Journal impact factors." Allergy. 1998 Dec;53(12):1225.