Understanding an Impact Factor

Anne Mueller, MA, OTS

Lab Coordinator/Research Assistant in Dr. Beth Fields’ Geriatric Health Services Research Lab

What is an impact factor?

The impact factor of a journal is a calculation of the number of articles cited in the previous two years divided by the number of citable articles the journal published in that time frame.1 It provides an approximate indication for how the journal is ranked in its field based on the extent of its readership and academic influence in recent years.1,2 If, for example, a journal has an impact factor of 2, that means all citable articles the journal published in the previous two years were cited an average of two times.3 Citations can appear either in articles from the same journal or, more often, from different journals.3

It is important to note that some journals use different types of impact factor measurements (such as ICV and CiteScore) that are not directly comparable to the journal impact factor.2 It is also important to note that some journals don’t have an impact factor, and impact factors have varied significance across disciplines.2

Where can I find journal impact factors?

Journal impact factors are published annually by Clarivate Analytics in the Journal Citation Reports (JCR).4 To access a journal’s impact factor, you will first need to register and login to the Web of Science Master Journal List. Once logged in, search for the journal of interest, find it in the results, and view the profile page. The impact factor will be located in the ‘Journal Metrics’ section. Additionally, impact factors can often be found on the journal’s webpage or publishing site.

Interpreting impact factors wisely

While an impact factor can be useful for assessing the influence of a journal, several other factors are important to consider when interpreting an impact factor.5 Some factors to consider include:

Publication date: the influence of articles that are cited over longer periods of time, rather than just in the last few years, is not captured by 2-year impact factors.3
Journal vs. article impact: the number of times each article is cited varies widely in any given journal – the journal impact factor does not accurately reflect the impact of each citable article.3 Some will naturally have much more impact than others!
Omitted review articles: the number of citable articles in a journal does not include review articles, but they are counted if they are cited, which can sometimes lead to an inflated impact factor.3
Bias towards journals that publish in English: the journals included in the Clarivate Analytics report are mostly those that are published in English, meaning that many international sources are excluded from the report.4
Exclusion of non-indexed journals: impact factor calculations are based on citations in indexed journals, which means that statistics from non-indexed journals and other sources of literature are excluded from these calculations.4