The Impact factor was devised by Eugene Garfield, the founder of the Institute for Scientific Information, now part of Thomson, a large worldwide US-based publisher. Impact factors are calculated each year by Thomson Scientific for those journals which it indexes, and the factors and indices are published in Journal Citation Reports. Some related values, also calculated and published by the same organization, are:
- the immediacy index: the number of citations the articles in a journal receive in a given year divided by the number of articles published.
- the cited half-life: the median age of the articles that were cited in Journal Citation Reports each year. For example, if a journal's half-life in 2005 is 5, that means the citations from 2001-2005 are half of all the citations from that journal in 2005, and the other half of the citations precede 2001.
- the aggregate impact factor for a subject category: it is calculated taking into account the number of citations to all journals in the subject category and the number of articles from all the journals in the subject category.
These measures apply only to journals, not individual articles or individual scientists (unlike, say, the H-index). The relative number of citations an individual article receives is better viewed as citation impact.
It is, however, possible to measure the Impact factor of the journals in which a particular person has published articles. This use is widespread, but controversial. Eugene Garfield warns about the "misuse in evaluating individuals" because there is "a wide variation from article to article within a single journal". Impact factors have a huge, but controversial, influence on the way published scientific research is perceived and evaluated.
The impact factor of a journal is calculated based on a three-year period. It can be viewed as an approximation of the average number of citations in a year, given to those papers in a journal that were published during the two preceding years. For example, the 2003 impact factor of a journal would be calculated as follows:
- A = the number of times articles published in 2001-2 were cited in indexed journals during 2003
- B = the number of "citable items" (usually articles, reviews, proceedings or notes; not editorials and letters-to-the-Editor) published in 2001-2
- 2003 impact factor = A/B
- (note that the 2003 impact factor was actually published in 2004, because it could not be calculated until all of the 2003 publications had been received.)
A convenient way of thinking about it is that a journal that is cited once, on average, for each article published has an IF of 1 in the expression above.
There are some nuances to this: ISI excludes certain article types (such as news items, correspondence, and errata) from the denominator. New journals, that are indexed from their first published issue, will receive an Impact Factor after the completion of two years' indexing; in this case, the citations to the year prior to Volume 1, and the number of articles published in the year prior to Volume 1 are known zero values. Journals that are indexed starting with a volume other than the first volume will not have an Impact Factor published until three complete data-years are known; annuals and other irregular publications, will sometimes publish no items in a particular year, affecting the count. The impact factor is for a specific time period; it is possible to calculate the impact factor for any desired period, for which the web site gives instructions. Journal Citation Reports includes a table of the relative rank of journals by Impact factor, in each specific science discipline, such as organic chemistry or psychiatry.
It is sometimes useful to be able to compare different journals and research groups. For example, a sponsor of scientific research might wish to compare the results to assess the productivity of its projects. An objective measure of the importance of different publications is then required and the impact factor (or number of publications) are the only ones publicly available. However, it is important to remember that different scholarly disciplines can have very different publication and citation practices, which affect not only the number of citations, but how quickly, after publication, most articles in the subject reach their highest level of citation. In all cases, it is only relevant to consider the rank of the journal in a category of its peers, rather than the raw Impact Factor value.
Impact factors are not infallible measures of journal qualitySeglen PO (1997). "Why the impact factor of journals should not be used for evaluating research". BMJ. 314 (7079): 498–502. PMID 9056804.. For example, it is unclear whether the number of citations a paper garners measures its actual quality or simply reflects the sheer number of publications in that particular area of research and whether there is a difference between them. Furthermore, in a journal which has long lag time between submission and publication, it might be impossible to cite articles within the three-year window. Indeed, for some journals, the time between submission and publication can be over two years, which leaves less than a year for citation. On the other hand, a longer temporal window would be slow to adjust to changes in journal impact factors. Thus, although the impact factor is appropriate for some fields of science such as molecular biology, it is not appropriate for subjects with a slower publication pattern, such as ecology. (It is possible to calculate the impact factor for any desired period, and the web site gives instructions.)
Favorable properties of the impact factor include:
- ISI's wide international coverage. Web of Knowledge indexes 9000 science and social science journals from 60 countries. This is perhaps only partially correct: see below.
- Results are widely (though not freely) available to use and understand.
- It is an objective measure.
- It has a wider acceptance than any of the alternatives.
- In practice, the alternative measure of quality is "prestige." This is rating by reputation, which is very slow to change, and cannot be quantified or objectively used. It merely demonstrates popularity.
The most commonly mentioned faults of the impact factor include:
- ISI's inadequate international coverage. Although Web of Knowledge indexes journals from 60 countries, the coverage is very uneven. Very few publications from languages other than English are included, and very few journals from the less-developed countries. Even the ones that are included are undercounted, because most of the citations to such journals will come from other journals in the same language or from the same country, most of which are not included.
- The failure to include many high quality journals in the applied aspects of some subjects, such as marketing communications, public relations and promotion management and many important but not peer-reviewed technical magazines. This editorial comment  of the Asian EFL Journal complains of Thomson / ISI's failure to even consider rating certain superior journals.
- The failure to incorporate book publications including textbooks, handbooks and reference books into the calculations of the impact factor.
- The number of citations to papers in a particular journal does not really directly measure the true quality of a journal, much less the scientific merit of the papers within it. It also reflects, at least in part, the intensity of publication or citation in that area, and the current popularity of that particular topic, along with the availability of particular journals. Journals with low circulation, regardless of the scientific merit of their contents, will never obtain high impact factors in an absolute sense, but if all the journals in a specific subject are of low circulation, as in some areas of botany and zoology, the relative standing is meaningful. Since defining the quality of an academic publication is problematic, involving non-quantifiable factors, such as the influence on the next generation of scientists, assigning this value a specific numeric measure cannot tell the whole story.
- The temporal window for citation is too short, as discussed above. Classic articles are cited frequently even after several decades, but this should not affect specific journals.
- In the short term - especially in the case of low-impact-factor journals - many of the citations to a certain article are made in papers written by the author(s) of the original article. This means that counting citations may be independent of the real “impact” of the work among investigators.
- The absolute number of researchers, the average number of authors on each paper, and the nature of results in different research areas, as well as variations in citation habits between different disciplines, particularly the number of citations in each paper, all combine to make impact factors between different groups of scientists incommensurable. Generally, for example, medical journals have higher impact factors than mathematical journals and engineering journals. This limitation is accepted by the publishers; it has never been claimed that they are useful between fields--such a use is an indication of misunderstanding.
- By merely counting the frequency of citations per article and disregarding the prestige of the citing journals, the impact factor becomes merely a metric of popularity, not of prestige.
- HEFCE was urged by the Parliament of the United Kingdom Committee on Science and Technology to remind Research Assessment Exercise (RAE) panels that they are obliged to assess the quality of the content of individual articles, not the reputation of the journal in which they are published .
Misuse of impact factor
- The impact factor is often misused to predict the importance of an individual publication based on where it was published. This does not work well since a small number of publications are cited much more than the majority - for example, about 90% of Nature's 2004 impact factor was based on only a fourth of its publications. The impact factor, however, averages over all articles and thus underestimates the citations of the top cited while exaggerating the number of citations of the average publication.
- Academic reviewers involved in programmatic evaluations, particularly those for doctoral degree granting institutions, often turn to ISI's proprietary IF listing of journals in determining scholarly output. This builds in a bias which automatically undervalues some types of research and distorts the total contribution each faculty member makes.
- The absolute value of an impact factor is meaningless. A journal with an IF of 2 would not be very impressive in Microbiology, while it would in Oceanography. Such values are nonetheless sometimes advertised by scientific publishers.
- The comparison of impact factors between different fields is invalid. Yet such comparisons have been widely used for the evaluation of not merely journals, but of scientists and of university departments. It is not possible to say, for example, that a department whose publications have an average IF below 2 is low-level. This would not make sense for Mechanical Engineering, where only two review journals attain such a value.
- Outside the sciences, impact factors are relevant for fields that have a similar publication pattern to the sciences (such as economics), where research publications are almost always journal articles, that cite other journal articles. They are not relevant for literature, where the most important publications are books citing other books. Therefore, ISI does not publish a JCR for the humanities.
- Even in the sciences, it is not fully relevant to fields, such as some in engineering, where the principal scientific output is conference proceedings , technical reports, and patents.
- Since only the ISI database journals are used, it undercounts the number of citations from journals in less-developed countries, and less-universal languages.
- Even though in practice they are applied this way, impact factors cannot correctly be the only thing to be considered by libraries in selecting journals. The local usefulness of the journal is at least equally important, as is whether or not an institution's faculty member is editor of the journal or on its editorial review board.
Manipulation of impact factors
A journal can adopt editorial policies that increase its impact factor. These editorial policies may not solely involve improving the quality of published scientific work. Journals sometimes may publish a larger percentage of review articles. While many research articles remain uncited after 3 years, nearly all review articles receive at least one citation within three years of publication, therefore review articles can raise the impact factor of the journal. The Thomson Scientific website gives directions for removing these journals from the calculation. For researchers or students having even a slight familiarity with the field, the review journals will be obvious.
Several methods, not necessarily with nefarious intent, exist for a journal to cite articles in the same journal which will increase the journal's impact factor.
Editorials in a journal do not count as publications. However when they cite published articles, often articles from the same journal, those citations increase the citation count for the article. This effect is hard to evaluate, for the distinction between editorial comment and short original articles is not obvious. "Letters to the editor" might refer to either class.
An editor of a journal may encourage authors to cite articles from that journal in the papers they submit. The degree to which this practice affects the citation count and impact factor included in the Journal Citation Reports cited journal data must therefore be examined. Most of these effects are thoroughly discussed on the site's help pages, along with ways for correcting the figures for these effects if desired. However, it is almost universal for articles in a journal to cite primarily its own articles, for those are the ones of the same merit in the same special field. If done artificially, the effect will become especially visible when (i) journals have a low impact factor (in absolute terms) and (ii) publish only few papers per year.
For example, we have analysed the citations of individual papers in Nature and found that 89% of last year’s figure was generated by just 25% of our papers. The most cited Nature paper from 2002−03 was the mouse genome, published in December 2002. That paper represents the culmination of a great enterprise, but is inevitably an important point of reference rather than an expression of unusually deep mechanistic insight. So far it has received more than 1,000 citations. Within the measurement year of 2004 alone, it received 522 citations. Our next most cited paper from 2002−03 (concerning the functional organization of the yeast proteome) received 351 citations that year. Only 50 out of the roughly 1,800 citable items published in those two years received more than 100 citations in 2004. The great majority of our papers received fewer than 20 citations.
This emphasizes the fact that the impact factor refers to the average number of citations per paper, and this is not a gaussian distribution. It is rather a Bradford distribution, as predicted by theory. Most papers published in a high impact factor journal will ultimately be cited many fewer times than the impact factor may seem to suggest, and some will not be cited at all. Therefore the Impact Factor of the source journal should not be used as a substitute measure of the citation impact of individual articles in the journal.
Also, researchers from UCLA have estimated that when scientists write up their work and cite other people's papers, only around 20% have read the original (based on the assumption that copying a reference implies not reading the original paper).
Use in scientific employment
Though the impact factor was originally intended as an objective measure of the reputability of a journal (Garfield), it is now being increasingly applied to measure the productivity of scientists. The way it is customarily used is to examine the impact factors of the journals in which the scientist's articles have been published. This has obvious appeal for an academic administrator who knows neither the subject nor the journals.
Other measures of impact
In 1976 Gabriel Pinski and Francis Narin suggested a recursive impact factor, to give citations from journals that have high impact greater weight than citations from low-impact journals. Such a recursive impact factor resembles the PageRank algorithm of the Google search engine, though the original Pinski and Narin paper uses a "trade balance" approach in which journals score highest when they are often cited but rarely cite other journals. A number of subsequent authors have proposed related approaches to ranking scholarly journals. In 2006, Johan Bollen, Marko A. Rodriguez, and Herbert Van de Sompel also proposed using the PageRank algorithm. From their paper:
|ISI Impact Factor||PageRank||Combined|
|1||52.28||ANNU REV IMMUNOL||16.78||Nature||51.97||Nature|
|2||37.65||ANNU REV BIOCHEM||16.39||Journal of Biological Chemistry||48.78||Science|
|3||36.83||PHYSIOL REV||16.38||Science||19.84||New England Journal of Medicine|
|4||35.04||NAT REV MOL CELL BIO||14.49||PNAS||15.34||Cell|
|5||34.83||New England Journal of Medicine||8.41||PHYS REV LETT||14.88||PNAS|
|6||30.98||Nature||5.76||Cell||10.62||Journal of Biological Chemistry|
|7||30.55||Nature Medicine||5.70||New England Journal of Medicine||8.49||JAMA|
|8||29.78||Science||4.67||Journal of the American Chemical Society||7.78||The Lancet|
|9||28.18||NAT IMMUNOL||4.46||J IMMUNOL||7.56||NAT GENET|
|10||28.17||REV MOD PHYS||4.28||APPL PHYS LETT||6.53||Nature Medicine|
The table shows the top 10 journals 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. That the New England Journal of Medicine is cited even more than Nature or Science might reflect the mix of review articles and original articles that it publishes. It is necessary to analyze the data for a journal in the light of a detailed knowledge of the journal literature.
- H-index, for the impact factor of individual scientists, rather than journals.
- Impact Factor, Immediacy Index, Cited Half-life
- Eugen Garfield (1998). "Der Impact Faktor und seine richtige Anwendung". Der Unfallchirurg. 101 (6): 413–414. Unknown parameter
- Maurice Bruynooghe Theory and Practice of Logic Programming and the ISI Web of Knowledge, Association for Logic Programming Newletter, November 2005
- S.A. Marashi. On the identity of “citers”: are papers promptly recognized by other investigators? (2005) Med. Hypotheses 65, 822. PubMed: 15990244.
- Fassoulaki A, Papilas K, Paraskeva A, Patris K (2002). "Impact factor bias and proposed adjustments for its determination". Acta anaesthesiologica Scandinavica. 46 (7): 902–5. doi:10.1034/j.1399-6576.2002.460723.x. PMID 12139549.
- Seglen PO (1997). "Why the impact factor of journals should not be used for evaluating research". BMJ. 314 (7079): 498–502. PMID 9056804.
- "Not-so-deep impact". Nature. 435 (7045): 1003–4. 2005. doi:10.1038/4351003a. PMID 15973362.
- Richard Monastersky (October 14 2005). "The Number That's Devouring Science". The Chronicle of Higher Education. Check date values in:
- M. V. Simkin and V. P. Roychowdhury (2003). "Read before you cite!". Complex Syst. 14: 269.
- Gabriel Pinski and Francis Narin (1976). "Citation influence for journal aggregates of scientific publications: Theory with application to literature of physics". Information Processing & Management. 12: 297–312.
- S. J. Liebowitz and J. P. Palmer. (1984). "Assessing the relative impacts of economics journals". Journal of Economic Literature. 22: 77–88.
- I. Palacios-Huerta and O. Volij (2004). "The measurement of intellectual influence". Econometrica. 72: 963–977. doi:10.1111/j.1468-0262.2004.00519.x.
- Y. K. Kodrzycki and P. D. Yu (2006). "New approaches to ranking economics journals". B. E. Journal of Economics Analysis and Policy. 5. doi:10.2202/1538-0645.1520.
- Johan Bollen, Marko A. Rodriguez, and Herbert Van de Sompel. (2006). "Journal Status". Scientometrics. 69 (3). Unknown parameter
- C. T. Bergstrom. (2007). "Eigenfactor: Measuring the value and prestige of scholarly journals". C&RL News. 68 (5). Unknown parameter
- The Impact Factor, from the Thomson ISI website.
- Epidemiology Inquiry 2007 Impact Factor listings for medical and science journals.
- The ASIS&T SIGMETRICS list is the principal venue for discussions on citation indexing.
- Scimago Journal & Country Rank SJR is an alternative indicator based on PageRank.
- List of ranking and impact factor by Sci-Bytes
- The correct use of impact factor (in French)
- H-index calculator for an individual scientist's impact
- Show me the data
- Estimated impact of publication venues in Computer Science
- Charles Jennings (1998). "Citation data: the wrong impact?". Nature Neuroscience. 1 (8): 641–642. Unknown parameter
- Somnath Saha, Sanjay Saint, and Dimitri A. Christakis (2003). "Impact factor: a valid measure of journal quality?". Journal of the Medical Library Association. 91 (1): 42–46. Unknown parameter
- Per O Seglen (1997). "Impact factor: a valid measure of journal quality?". British Medical Journal. 314: 497. Unknown parameter
- C. Judson King et al (2006-07-27). "Scholarly Communication: Academic values and sustainable models".
- Philip Ball. "Prestige is factored into journal ratings". Nature 439, p.770-771.
- Publish or Perish calculates various statistics, including the h-index and the g-index using Google Scholar data.
- Hubert Chanson (2008). Some Experience with Digital Publishing, Open Access Repository, Research Impact and Quality. In: Open Access Collections workshop, Australian Partnership for Sustainable Repositories, Brisbane QLD, Australia, 14 February, 2008
- Hubert Chanson (2007). "Research Quality, Publications and Impact in Civil Engineering into the 21st Century. Publish or Perish, Commercial versus Open Access, Internet versus Libraries ?" Canadian Journal of Civil Engineering, 34 8: 946-951