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Journal Impact Factor

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Research cannot be done in isolation: for research to have an impact and for the researcher to contribute to and learn from the larger research community, it is essential that research findings be shared. This is usually done by publishing the research and its findings in a subject-related journal.

As a young researcher in the field, it is naturally important to you that your paper be read by the largest audience, so as to increase your recognition in the field by your peers. There are two ways of doing this: (1) by picking a journal that publishes papers in the format you have written your manuscript in (e.g., reviews/case reports/original research) and (2) by picking a journal that will provide the widest audience for your work.

One criterion used by many researchers to select the journal they feel will be of maximum benefit to them in terms of recognition of their work is the journal Impact Factor. We discuss in this article the history of the journal Impact Factor, how to calculate it, why it is used, the advantages and disadvantages of relying on journal Impact Factors, and alternative ranking methods. A list of references with online links is given at the end.

Dr. Eugene Garfield is the Founder and Chairman Emeritus of the Institute for Scientific Information (ISI). The concept of an Impact Factor was first proposed by him1 in an article in Science in 1955. In the sixties, Sher and Garfield2 created the 'journal Impact Factor' to help select journals for the Science Citation Index. The ISI began to publish Journal Citation Reports in 1975 as part of the Science Citation Index (SCI) and Social Sciences Citation Index (SSCI).

What is journal Impact Factor?
The journal Impact Factor is a technique used to quantitatively assess the 'impact' of a journal in a particular subject. It is also used as a tool to compare different journals with each other. The Impact Factor measures the number of times a journal has been cited over a particular period. In other words, it measures how frequently the articles in a particular journal have been cited within a specified period of time.

The Impact Factor removes some of the bias of citation counts which favor large journals over small ones, frequently issued journals over less frequently issued ones, and older journals over newer ones. Especially in older journals, there is a larger citable body of literature than in younger journals. All things being equal, the larger the number of previously published articles, the more often a journal will be cited.

Calculation of journal Impact Factor
The journal Impact Factor is basically a ratio, calculated3 by dividing the total number of citations made in a given year to any article in the selected journal by the total number of citable articles published in that journal over a defined timeframe. This timeframe is usually the 2 years immediately preceding the selected year.

Therefore, the Impact Factor for a journal X for the year 2006 would be calculated as follows:

Impact FactorX    =
All citations in 2006 to articles published in X in 2004 and 2005
All citable articles published in X in 2004 and 2005
Where can I find out the Impact Factor of a journal?
Journal Impact Factors are calculated each year by the ISI for the journals which it indexes (in the SCI and SSCI) and they are published annually in Journal Citation Reports (JCR) (URL:

You can see a listing of journal Impact Factors at the links given below:
for 2005: (Excel file)

As you must have understood, the journal Impact Factors for 2006 will be available only in 2007, as the citation figures for all journals will be completed only at the end of the year.
What is the relevance of journal Impact Factor to researchers?
Journal Impact Factors can be used for the following functions:
  • To decide which journal is the best one for the publication of your paper,
    you may wish to compare the Impact Factors of different journals in your subject area. Since there are a vast number of journals to choose from, the Impact Factor provides a more objective measure of the quality of work previously published in that journal and the reach of the publication. As a general rule, journals with high Impact Factors are considered more prestigious. However, some librarians argue that the numerator in the Impact Factor calculation is, by itself, even more relevant.
  • Academic evaluation. It is now common to use Impact Factors in the process of academic evaluation of a researcher for tenure, grants, funding, etc. The Impact Factor can be used to provide a rough estimate of the prestige of journals in which the researcher's work has been published. This is ideally done with the simultaneous inclusion of other factors in the assessment such as productivity, individual citation rate of the researcher's work, review committee opinion, etc. Academic evaluation should not lay undue stress on journal Impact Factor, as explained below.
  • To compare different research groups. The Impact Factor can also be used to compare the work being done by different research groups. For example, you might wish to compare the results of different research teams experimenting with the same drug. One way of doing so is by assessing how important their published research is to the scientific community. An objective measure of the importance of publications on the same drug by different researchers is then required and the journal Impact Factor is one that is readily available.
  • To select journals for libraries. Today the number of peer-reviewed research publications is estimated to exceed 16,000 worldwide, which makes it difficult for a single library to include all of them in its collection. The Impact Factor provides library administrators with a tool to decide which journals to retain in their collections and which new ones to acquire for their libraries.

Advantages of journal Impact Factor
The advantages of knowing and using journal Impact Factors are given below4:
  • The ISI's wide international coverage. The Web of Science ( set up by ISI indexes approximately 8,700 of the most prestigious, high impact research journals in the world.
  • Results are easily available to use and understand.
  • Journal Impact Factors are more widely accepted than the available alternatives.
  • Instead of an arbitrary ranking of a journal as 'prestigious,' the journal Impact Factor makes it possible to provide a quantitative, and hence more objective, measure of the quality of papers published in a particular journal.

Drawbacks of journal Impact Factor

The most commonly mentioned faults of journal Impact Factors are given below 4:
  • The Web of Knowledge5 (URL: database set up by the ISI tries to provide an extensive coverage of publications from around the world. However, the actual coverage provided by the Web of Knowledge is somewhat unevenly distributed. Although it indexes journals from 60 countries, there are not many publications from under-developed countries and very few in languages other than English.
  • The number of citations to a particular journal does not really indicate the true standard of the journal, or the scientific merit of the articles published in it. In reality, it demonstrates the current popularity of a particular research topic and the extent of distribution of the journal. Journals with a low circulation, e.g., super-specialty or disease-specific journals, will never obtain high Impact Factors6. However, when all the journals in a specific subject are being compared, the use of journal Impact Factor provides a more meaningful comparison.
  • The time period for citations, just 2 years, is too short. Classic articles continue to be cited frequently, even after several decades.
  • The absolute number of researchers, the average number of authors on each paper, the nature of results in different research areas, and the variations in citation habits between different disciplines, all combine to make it difficult to compare journal Impact Factors between different groups of scientists. Generally, for example, medical journals have higher Impact Factors than mathematical journals and engineering journals. Journal Impact Factors should, therefore, not be used for the comparative evaluation of different scientists and university departments for funding or other purposes.
  • The absolute value of an Impact Factor is meaningless. As explained in point #2, the journal Impact Factor value can be influenced by the subject area of the journal. For example, a journal with an Impact Factor of 2 would not be very impressive in a subject like Microbiology, but it would be in Oceanography.
  • In a field of study outside of the sciences (i.e., what is termed STM in the publishing world), journal Impact Factors can be calculated and used only for subject areas where the publication pattern is similar to that of the sciences. That is to say, research is almost always presented in the form of journal articles that cite other journal articles as references, e.g., in Economics. Journal Impact Factors are not relevant for subjects like English literature where the important publications are usually books, citing other books. That is the reason why the ISI does not publish a JCR for the humanities.
  • Even in the sciences, the journal Impact Factor is not completely relevant in certain fields, e.g., some branches of Engineering, where conference proceedings, technical reports, and patents form the principal scientific output.
Misuse of journal Impact Factor
Journal Impact Factors, like all tools, must be used wisely and not relied on exclusively. When forming an opinion about a journal or researcher, a number of factors need to be considered without giving undue emphasis to any one factor.
  • Journal Impact Factors have been criticized in recent years because institutions, funding agencies, and scientific committees have been increasingly using them to evaluate individual researchers. Instead of obtaining actual citation counts for articles written by the persons they are assessing, they look at the Impact Factors of the journals in which the articles have been published2. The argument used is that recently published articles may not have had enough time to be cited, so the journal Impact Factor is used to give an indication of the expected impact of the article. In other words, the journal Impact Factor is used as a tool to predict the possible citation rate ('Impact Factor') of an individual article. This can be rather risky: the journal may well be high impact, but the published article may not make any impact at all (i.e., have a low citation rate). In an assessment done by P.O. Seglen7, it was shown that 90% of a journal's Impact Factor is derived from just 50% of the cited articles.
  • In Finland, there appears to be an extreme dependence on journal IF8. The allotment of funds to university hospitals by the Finnish government partially depends on 'publication points,' which are derived from the Impact Factor of journals in which the hospital researchers publish their work. If the work has been recently published in a high impact journal with high editorial standards and strict peer review, it is assumed that the work is also of high standard.
  • Another area of misuse of journal Impact Factors is the area of self-citations. In an attempt to increase their Impact Factor, some journals might request authors to cite articles previously published in the journal, prior to accepting the manuscript for publication.
  • In an article in the BMJ9 , Richard Smith presents the case of a journal whose editors wrote to the authors of a submitted manuscript, asking them to increase the number of references in their manuscript to research papers already published in that journal. This would have contributed to increasing the journal's Impact Factor. This was clearly a blatant example to increase the journal's prestige by manipulating the journal's Impact Factor and would greatly distort the true value of that journal.
  • Journals today also realize the importance given by academicians to a high journal Impact Factor. Therefore, many journals today publish only those articles which they feel will be cited by others later on, thus contributing to the journal's Impact Factor. Some observers point out that this may mean that journals will be designed for citing rather than reading and for authors (who can cite) rather than readers (who cannot)10.

Alternatives to journal Impact Factor
The widespread use of journal Impact Factors and their increasing importance in research assessment, along with all the flaws detailed above, has prompted researchers in the field of Scientometrics to propose alternative methods for journal assessment.

When ranking all the journals pertaining to a particular subject, it is inadequate to only compare the Impact Factor without consideration of subject bias. Hirst introduced what he called the Disciplinary Impact Factor (DIF) to overcome this subject bias. It is based on the number of times a journal is cited within a particular subject area rather than the entire SCI database8.

In 2006, Johann Bollen et al recommended using the PageRank algorithm (utilized by Google), to distinguish the 'quality' of citations and hence improve Impact Factor calculation4.

ISI Impact Factor
Annu Rev Immunol
Annu Rev Biochem
Physiol Rev
Nat Rev Mol Cell Bio
New Engl J Med
Nat Med
Scie nce
Nat Immunol
Rev Mod Phys
J Biol Chem
Phys Rev Lett
New Engl J Med
J Am Chem Soc
J Immunol
Appl Phys Lett
ISI Impact Factor
New Engl J Med
J Biol Chem
Nat Genet
Nat Med

The table from Wikipedia4 shows the top 10 journals, as ranked by ISI Impact Factor, PageRank, and a modified system that combines the two (based on 2003 data). Nature and Science are generally regarded as the most prestigious journals and in the combined system they come out on top. You will notice that the New England Journal of Medicine is cited even more than Nature or Science. This could be because it publishes a mix of review articles, original research, and correspondence which makes it more broad based and thus it reaches out to a larger audience.
The journal impact factor is a very useful tool for the evaluation of journals, but it must be used wisely. When deciding which journal to send your manuscript to, it is important to keep in mind the content of the journal (i.e., does it contain only reviews, does it have a mix of original research and reviews, does it include correspondence, is a very broad range of topics covered), the field of study, and the academic merit of the individual articles published in the journal. It is important to remember that a journal with a very narrow focus (e.g., one dealing with a very specialized subject) may have a lower Impact Factor than a more broad based one. Due to their inherent drawbacks, as detailed above, journal Impact Factors should not be given too much weightage when assessing a person for tenure or grants, and should only form one part of the total assessment.