Using Journal Citation Reports Wisely
You should not depend solely on citation data in your journal evaluations. Citation data are not meant to replace informed peer review. Careful attention should be paid to the many conditions that can influence citation rates such as language, journal history and format, publication schedule, and subject specialty.
The number of articles given for journals listed in JCR include primarily original research and review articles. Editorials, letters, news items, and meeting abstracts are usually not included in article counts because they are not generally cited. Journals published in non-English languages or using non-Roman alphabets may be less accessible to researchers worldwide, which can influence their citation patterns. This should be taken into account in any comparative journal citation analysis.
You should also consider the following four conditions, which may affect journal's ranking and Impact Factor.
Impact Factor by Article Type
Clarivate Analytics manually codes each published article with a document type, but it is not feasible to individually code the millions of references processed each year. Therefore, citation counts in JCR do not distinguish between citations to letters, reviews, or original research articles, even though only original research and review articles are used in impact factor calculations. If a journal publishes a large number of letters one year, there may be a temporary increase in the number of citations received. This increase is not proportionately reflected in the JCR article count given. To identify and evaluate any such phenomena, detailed article-by-article analyses can be conducted.
Changes in Journal Format
Sudden changes in a journal's size can affect the Impact Factor. The average number of cites per article is lowered when there are more one-year-old articles than two-year-old articles because article citation rates tend to increase in the second year after publication. Likewise, when an article count drops, the Impact Factor may rise temporarily. The article counts used to calculate the Impact Factor are provided, so that any sudden changes can be noted.
Title Changes and Citation Metrics
After a title change, two JCR years must pass before the new title fully replaces the previous title in JCR. In the first year after a journal title change, the new title is listed with an Immediacy Index but no impact factor because the article count for the two preceding years, used in Impact Factor calculations, is zero. The superseded title is listed with a normal two-year Impact Factor. One year later, JCR lists separate impact factors for the new title and for the superseded title, but only the new title will have an Immediacy Index. In this second year, the Impact Factor for a new title may be lower than expected because the article count includes only earlier articles. Similarly, the Impact Factor for the superseded title may be higher than expected because it is based upon only older articles. To calculate a unified Impact Factor, you can total the cites to the two previous years and divide that by the sum of the article counts for the two titles. For a listing of journal title changes, where both the new title and the superseded title appear in JCR, see the Journal Title Changes page, which is accessible from the Journal page, the Journal Search page and the Summary List page.
Cited-only Journals in JCR
Some of the journals listed in JCR are not citing journals, but are cited-only journals. A journal that has no Citing Journal page information is a cited-only journal. Cited-only journals were not indexed as source items. They may represent former titles, titles that have been removed from coverage, or titles that are not selected for coverage in Clarivate Analytics citation databases.
This is significant when comparing journals because self-citations from cited-only journals are not included in the JCR data. Self-citations represent a significant percent of the citations that a journal receives. Evaluations including cited-only journals are enhanced by self-citation analysis.