When researchers refer to another author’s work in their own published work, they cite it.
Such citations can be analysed to measure the usage of the cited work.
The impact factor is one of three measures created by the Institute for Scientific Information (ISI) which gauge the way a journal receives citations to its articles over time. The other two measures are the immediacy index and cited half-life (Note: these two measures are not discussed in this guide).
- What are impact factors?
- The Impact factor formula
- How to use impact factors
- Deciding where to publish
- Be aware that…
- Alternatives to impact factors
- Related guides
What are impact factors?
Impact factors provide a quantitative method of evaluating and comparing journals within a subject category.
Impact factors measure the frequency with which the "average article" in a journal has been cited in a particular year or period.
Impact factors are derived from citation indexes owned by Thomson Scientific:
- Science Citation Index ®
- Social Sciences Citation Index ®
- Arts & Humanities Citation Index ®.
Algorithms are used to analyse the data in these indexes and produce Journal Citation Reports (JCR ®).
JCR® is the sole place where impact factors are calculated and reported. JCR® covers approximately 7,500 journal titles. This is a small minority of the journals in publication, so be aware: most journals do not have an impact factor.
Read about Thomson Scientific's journal selection process.
The Impact factor formula
ISI defines the impact factor as a ratio of two elements. The denominator is the total number of "citable" articles published in a particular journal within a given timeframe. The numerator is the total number of citations in the current year to any article published in this journal during that given timeframe. ISI has defined this time frame as two years.
The impact factor (IF) of journal A in a particular year Y is calculated using the following formula:
How to use impact factors
Impact factors can be used to:
- identify journals in which to publish
- identify journals relevant to your research
- confirm the status of journals in which you have published
But what’s in a number? To state that the British Medical Journal (BMJ) has an impact factor of 9.052 is reasonably meaningless.
It is more meaningful to say that BMJ’s impact factor ranks sixth of 105 journals in the field of general and internal medicine. Or to compare the journal’s impact factor of 9.052 with the aggregate impact factor for its field: 4.326.
It is recommended therefore that the impact factor for a journal is not looked at in isolation. Rather, the impact factor of a journal should be compared to the impact factors for other journals within the same subject category.
Watch Thomson Scientific’s demonstration of Journal Citation Reports - the impact factor.
Deciding where to publish
- Target a journal with a high impact factor – or, in fact, with any impact factor at all!
- Consider the international reach and inclusion of the abstracting and indexing services in relation to the journal you are targeting. You need other researchers to be able to find your work. Is the journal indexed by the most significant bibliographic databases in your subject discipline?
- When deciding between journals, consider circulation numbers and potential readership.
Contact your supervisor, colleagues or Liaison Librarian for assistance in identifying this information.
Be aware that...
- Many journals do not have an impact factor.
- The impact factor cannot assess the quality of individual articles. Even if citations were evenly distributed among articles, the impact factor would only measure the interests of other researchers in an article, not its importance and usefulness.
- Only research articles, technical notes and reviews are “citable” items. Editorials, letters, news items and meeting abstracts are “non-citable items”.
- Only a small percentage of articles are highly cited and they are found in a small subset of journals. This small proportion accounts for a large percentage of citations.
- Controversial papers, such as those based on fraudulent data, may be highly cited, distorting the impact factor of a journal.
- Citation bias may exist. For example, English language resources may be favoured. Authors may cite their own work.
Alternatives to impact factors
It should be noted that impact factors are a quantitative method of evaluating journals and are by no means a substitute for qualitative measures such as peer-review.
For many disciplines, JCR® has limited coverage. Do not despair however - there are many other resources to assist with journal evaluation, including:
Journal Quality List (Business)
The Journal Quality List compiled and edited by Dr. Anne-Wil Harzing, covers the areas of economics, finance, accounting, management, and marketing.
Association for Information Systems : Journal Rankings
The Association for Information Systems' Journal Rankings edited by Carol Saunders, covers the areas of management information systems.
Contact your Liaison Librarian for assistance in determining relevant resources for your discipline.
Related guides