Bibliometrics: Journal Impact Factor
The average citation level of a journal is an extremely limited indicator, and is not a replacement for expert, qualitative assessment of the journal. The distribution of citations is typically skewed, therefore using the mean is problematic. Many articles published in "high impact" journals receive very few or indeed no citations at all. Many factors can influence citation rates such as language, journal history, publication schedule, and subject specialty and scope.
Other factors which can affect a journal's ranking or metrics include:
- article type(s)
- changes in journal format
- title changes
- cited-only journals (i.e. whereby references from a journal (including self-citations) are not included in the dataset)
Many of the journals in Web of Science and Scopus are English-language from North America, Western Europe and Australia. Irish journals in many research areas can be underrepresented.
Review journals and original research journals are treated the same even though the citation patterns of these types of articles differ.
For comparing journals specializing in cutting-edge research, the immediacy index can provide a useful perspective.
The Immediacy Index is the average number of times an article is cited in the year it is published.
- The journal Immediacy Index indicates how quickly articles in a journal are cited.
- The aggregate Immediacy Index indicates how quickly articles in a subject category are cited.
The Immediacy Index is calculated by dividing the number of citations to articles published in a given year by the number of articles published in that year.
What is the Journal Impact Factor?
The journal Impact Factor is the average number of times articles from the journal published in the past two years have been cited in the Journal Citation Reports (JCR) year.
The Impact Factor is calculated by dividing the number of citations in the JCR year by the total number of articles published in the two previous years. An Impact Factor of 1.0 means that, on average, the articles published one or two year ago have been cited one time. An Impact Factor of 2.5 means that, on average, the articles published one or two year ago have been cited two and a half times. Citing articles may be from the same journal; most citing articles are from different journals.
The journal Impact Factor was developed by Eugene Garfield at the Institute for Scientific Information (ISI), and is now owned by Clarivate Analytics.
Impact Factor uses Web of Science (Clarivate Analytics) citation data.
Example of a Journal Impact Factor
Calculation of the 2014 Journal Impact Factor for the Journal of Information Technology
San Francisco Declaration on Research Assessment
Initiated in 2012, the American Society for Cell Biology and a group of editors and publishers of scholarly journals drafted and circulated a declaration that recognises the need to improve the way in which the outputs of scientific research are evaluated.
"The Journal Impact Factor, as calculated by Thomson Reuters, was originally created as a tool to help librarians identify journals to purchase, not as a measure of the scientific quality of research in an article. With that in mind, it is critical to understand that the Journal Impact Factor has a number of well-documented deficiencies as a tool for research assessment. These limitations include:
- citation distributions within journals are highly skewed
- the properties of the Journal Impact Factor are field-specific: it is a composite of multiple, highly diverse article types, including primary research papers and reviews
- Journal Impact Factors can be manipulated (or "gamed") by editorial policy
- data used to calculate the Journal Impact Factors are neither transparent nor openly available to the public."