A closer look at the Journal Impact Factor numerator

Citation is a means of acknowledging the influence and relevance of the work of others within a scholarly publication. Since each citation is an acknowledgement of influence it follows that any metric designed to comprehensively measure journal influence should include all citations made to the journal title. The Journal Impact Factor in Journal Citation Reports from Clarivate Analytics is the only metric that accomplishes this task.

The Journal Impact Factor is the brainchild of our late founder, Dr. Eugene Garfield, and has been the mainstay of journal analysis since its inception. Here, we will look at the calculation of the Journal Impact Factor and take a deeper dive into what makes up the numerator of the calculation.

The Journal Impact Factor is defined as citations to the journal in the current year to items in the previous two years divided by the count of scholarly items in those previous two years.


The citations that comprise the Journal Impact Factor numerator are drawn from the premier journal and proceedings indexes in Web of Science (Science Citation Index Expanded, Social Sciences Citation Index, Arts & Humanities Citation Index, and both the Science and Social Science and Humanities editions of the Conference Proceedings Citation Index, as well as the Emerging Sources Citation Index). Citations from the Book Citation Index do not contribute toward JCR metrics.

The numerator of the Journal Impact Factor consists of any citation to the journal as defined by the title of the journal, irrespective of what item in the journal might be cited. JCR therefore aggregates all citations to a given journal in the numerator regardless of cited document type.

Citations with incomplete article-level metadata or errors cannot be counted in metrics that are based on article-level data. Therefore, when using one of these metrics to gauge journal performance there will be pieces missing from the overall picture.

Some cited references will not specify an issue number, page, or article identifier. In the example below, since the author Christina Gross published two articles in the journal Cell Reports in the year 2015, it’s impossible to determine which article the citation on the top line is referencing:

Similarly, some citations will specify incorrect metadata such as volume or page.  These citations also can’t be linked at the article level. The citation shown here appears to list an incorrect page number, and as a consequence it is impossible to link to any of the three articles published by the author Kim H in 2015 in the journal Cell Reports:

There is a potential for introduction of metadata errors at various stages of the publication process—author, publisher, indexer, etc.

The Journal Impact Factor is truly unique among journal metrics in that all citations to a journal title are counted. The Journal Impact Factor is a journal-level metric that counts all recognizable citations to a journal title, even in cases where there is no information in the reference about which particular article has been cited, such as the papers shown in the following table:

The Journal Impact Factor therefore gives you a more complete picture of journal-level citation and enables a more accurate assessment of journal performance.

No matter how simple and unique a journal title may be, there will always be variations in how the journal is referenced. Journals have different policies covering citation style, and citing authors can further alter the title by abbreviating or truncating it in different ways. Some journals may also have acronyms that are in use among its close followers.

Journal Citation Reports is designed to be as inclusive as possible when gathering and counting the citations for the Journal Impact Factor and other metrics.  Any recognizable variant of the title is included. In addition to the preferred journal abbreviation, Journal Citation Reports will consider alternate spellings, truncations, and acronyms. When the Journal Citations Reports process parses the reference, the cited work is compared to our variant dictionary, which contains over 250,000 variant titles for approximately 30,000 currently active and historic journal titles. Here is a snippet from the variant dictionary for the title Cancer and Metastasis Reviews:

Even citations with no article-level identifying characteristics will be counted as long as the cited work is a clear variant of the journal title.


Because it is more inclusive by design, the Journal Impact Factor allows you to get the full picture of journal performance.