Impact factor

Impact factor

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Encyclopedia
The impact factor, often abbreviated IF, is a measure reflecting the average number of citation
Citation
Broadly, a citation is a reference to a published or unpublished source . More precisely, a citation is an abbreviated alphanumeric expression Broadly, a citation is a reference to a published or unpublished source (not always the original source). More precisely, a citation is an abbreviated...

s to articles published in science and social science journals
Scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...

. It is frequently used as a proxy
Proxy (statistics)
In statistics, a proxy variable is something that is probably not in itself of any great interest, but from which a variable of interest can be obtained...

 for the relative importance of a journal within its field, with journals with higher impact factors deemed to be more important than those with lower ones. The impact factor was devised by Eugene Garfield
Eugene Garfield
Eugene "Gene" Garfield is an American scientist, one of the founders of bibliometrics and scientometrics. He received a PhD in Structural Linguistics from the University of Pennsylvania in 1961. Dr. Garfield was the founder of the Institute for Scientific Information , which was located in...

, the founder of the Institute for Scientific Information
Institute for Scientific Information
The Institute for Scientific Information was founded by Eugene Garfield in 1960. It was acquired by Thomson Scientific & Healthcare in 1992, became known as Thomson ISI and now is part of the Healthcare & Science business of the multi-billion dollar Thomson Reuters Corporation.ISI offered...

 (ISI), now part of Thomson Reuters
Thomson Reuters
Thomson Reuters Corporation is a provider of information for the world's businesses and professionals and is created by the Thomson Corporation's purchase of Reuters Group on 17 April 2008. Thomson Reuters is headquartered at 3 Times Square, New York City, USA...

. Impact factors are calculated yearly for those journals that are indexed in Thomson Reuters
Thomson Reuters
Thomson Reuters Corporation is a provider of information for the world's businesses and professionals and is created by the Thomson Corporation's purchase of Reuters Group on 17 April 2008. Thomson Reuters is headquartered at 3 Times Square, New York City, USA...

 Journal Citation Reports
Journal Citation Reports
Journal Citation Reports is an annual publication by the Healthcare & Science division of Thomson Reuters. It has been integrated with the Web of Knowledge, by Thomson Reuters, and is accessed from the Web of Science to JCR Web. It provides information about academic journals in the sciences and...

.

Calculation


In a given year, the impact factor of a journal is the average number of citations received per paper published in that journal during the two preceding years. For example, if a journal has an impact factor of 3 in 2008, then its papers published in 2006 and 2007 received 3 citations each on average in 2008. The 2008 impact factor of a journal would be calculated as follows:
A = the number of times articles published in 2006 and 2007 were cited by indexed journals during 2008.
B = the total number of "citable items" published by that journal in 2006 and 2007. ("Citable items" are usually articles, reviews, proceedings, or notes; not editorials or Letters-to-the-Editor.)

2008 impact factor = A/B.


(Note that 2008 impact factors are actually published in 2009; they cannot be calculated until all of the 2008 publications have been processed by the indexing agency.)

New journals, which are indexed from their first published issue, will receive an impact factor after two years of 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 get an impact factor until they have been indexed for three years. Annuals and other irregular publications sometimes publish no items in a particular year, affecting the count. The impact factor relates to a specific time period; it is possible to calculate it for any desired period, and the Journal Citation Reports (JCR) also includes a 5-year impact factor. The JCR shows rankings of journals by impact factor, if desired by discipline, such as organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 or psychiatry
Psychiatry
Psychiatry is the medical specialty devoted to the study and treatment of mental disorders. These mental disorders include various affective, behavioural, cognitive and perceptual abnormalities...

.

Use


The IF is used to compare different journals within a certain field. The ISI Web of Knowledge indexes more than 11,000 science and social science journals.

Criticisms


Numerous criticisms have been made of the use of an impact factor, including the more general debate on the usefulness of citation metrics. Criticisms mainly concern the validity of the impact factor, policies that alter it, and its incorrect application.

Validity

  • The impact factor is highly discipline-dependent. The percentage of total citations occurring in the first two years after publication varies highly among disciplines from 1-3 percent in the mathematical and physical sciences to 5-8 percent in the biological sciences.
  • The impact factor could not be reproduced in an independent audit.
  • The impact factor refers to the average number of citations per paper, but this is not a normal distribution. It is rather a Bradford distribution
    Bradford's law
    Bradford's law is a pattern first described by Samuel C. Bradford in 1934 that estimates the exponentially diminishing returns of extending a search for references in science journals...

    , as predicted by theory. Being an arithmetic mean
    Arithmetic mean
    In mathematics and statistics, the arithmetic mean, often referred to as simply the mean or average when the context is clear, is a method to derive the central tendency of a sample space...

    , the impact factor therefore is not a valid representation of this distribution and unfit for citation evaluation.
  • 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. Garfield, however, maintains that this phenomenon hardly influences a journal's impact factor. Moreover, a study of author self-citations in diabetes literature found that the frequency of author self-citation was not associated with the quality of publications. Similarly, journal self-citation is common in journals dealing in specialized topics having high overlap in readership and authors, and is not necessarily a sign of low quality or manipulation.
  • Journal ranking lists constructed based on the impact factor only moderately correlate with journal ranking lists based on the results of an expert survey
    Statistical survey
    Survey methodology is the field that studies surveys, that is, the sample of individuals from a population with a view towards making statistical inferences about the population using the sample. Polls about public opinion, such as political beliefs, are reported in the news media in democracies....

    .

Editorial policies which affect the impact factor


A journal can adopt editorial policies that increase its impact factor.
  • Journals may publish a larger percentage of review articles which generally are cited more than research reports. Therefore review articles can raise the impact factor of the journal and review journals will therefore often have the highest impact factors in their respective fields. Conversely, journals may choose not to publish minor articles, such as case reports in medical journals, which are unlikely to be cited and would reduce the average citation per article.
  • Journals may change the fraction of "citable items" compared to front-matter in the denominator of the IF equation. Which types of articles are considered "citable" is largely a matter of negotiation between journals and Thomson Scientific. As a result of such negotiations, impact factor variations of more than 300% have been observed. For instance, editorials in a journal are not considered to be citable items and therefore do not enter into the denominator of the impact factor. However, citations to such items will still enter into the numerator, thereby inflating the impact factor. In addition, if such items cite other articles (often even from the same journal), those citations will be counted and will increase the citation count for the cited journal. This effect is hard to evaluate, for the distinction between editorial comment and short original articles is not always obvious. "Letters to the editor" might refer to either class.
  • 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.

Manipulations of the impact factor

  • In 2007, a specialist journal with an impact factor of 0.66 published an editorial that cited all its articles from 2005 to 2006 in a protest against the absurd use of the impact factor. The large number of citations meant that the impact factor for that journal increased to 1.44. As a result of the increase, the journal was not included in the 2008 and 2009 Journal Citation Reports.
  • In 2008, a single article "A short history of SHELX" included a sentence that essentially instructs readers to cite the paper: "This paper could serve as a general literature citation when one or more of the open-source SHELX programs (and the Bruker AXS version SHELXTL) are employed in the course of a crystal-structure determination". This article received more than 6,600 citations. As a consequence, the impact factor of the journal Acta Crystallographica Section A rose from 2.051 in 2008 to 49.926 in 2009, more than Nature (31.434) and Science (28.103). The second most cited article in Acta Crystallographica Section A in 2008 had only 28 citations.

Incorrect application of impact factor

  • The IF may be incorrectly applied to evaluate the significance of an individual publication or to evaluate an individual researcher.

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 quarter of its publications, and thus the importance of any one publication will be different from, and in most cases less than, the overall number. The impact factor, however, averages over all articles and thus underestimates the citations of the most cited articles while exaggerating the number of citations of the majority of articles. Consequently, the Higher Education Funding Council for England
Higher Education Funding Council for England
The Higher Education Funding Council for England is a non-departmental public body of the Department for Business, Innovation and Skills in the United Kingdom, which has been responsible for the distribution of funding to Universities and Colleges of Higher and Further Education in England since...

 was urged by the House of Commons Science and Technology Select Committee
Science and Technology Select Committee
The Science and Technology Select Committee is a select committee of the House of Commons in the Parliament of the United Kingdom.The original Science and Technology was abolished upon the creation of the Innovation, Universities, Science and Skills Committee on November 6, 2007...

 to remind Research Assessment Exercise
Research Assessment Exercise
The Research Assessment Exercise is an exercise undertaken approximately every 5 years on behalf of the four UK higher education funding councils to evaluate the quality of research undertaken by British higher education institutions...

 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.

Responses

  • Because "the impact factor is not always a reliable instrument" in November 2007 the European Association of Science Editors (EASE)
    European Association of Science Editors
    The ' is a non-profit membership organisation for people interested in science communication and editing. Founded in 1982, in France, EASE now has an international membership from diverse backgrounds and professional experience....

     issued an official statement recommending "that journal impact factors are used only - and cautiously - for measuring and comparing the influence of entire journals, but not for the assessment of single papers, and certainly not for the assessment of researchers or research programmes".
  • In July 2008, the International Council for Science
    International Council for Science
    The International Council for Science , formerly the International Council of Scientific Unions, was founded in 1931 as an international non-governmental organization devoted to international co-operation in the advancement of science...

     (ICSU) Committee on Freedom and Responsibility in the conduct of Science (CFRS) issued a "Statement on publication practices and indices and the role of peer review in research assessment", suggesting some possible solutions, e.g. considering penalising scientists for an excessive number of publications per year.
  • In February 2010, the Deutsche Forschungsgemeinschaft
    Deutsche Forschungsgemeinschaft
    The Deutsche Forschungsgemeinschaft is an important German research funding organization and the largest such organization in Europe.-Function:...

     (German Foundation for Science) published new guidelines to evaluate only articles and no bibliometric information on candidates to be evaluated in all decisions concerning "...performance-based funding allocations, postdoctoral qualifications, appointments, or reviewing funding proposals, [where] increasing importance has been given to numerical indicators such as the h-index
    H-index
    The h-index is an index that attempts to measure both the productivity and impact of the published work of a scientist or scholar. The index is based on the set of the scientist's most cited papers and the number of citations that they have received in other publications...

     and the impact factor". This decision follows similar ones of the National Science Foundation
    National Science Foundation
    The National Science Foundation is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health...

     (US) or the Research Assessment Exercise
    Research Assessment Exercise
    The Research Assessment Exercise is an exercise undertaken approximately every 5 years on behalf of the four UK higher education funding councils to evaluate the quality of research undertaken by British higher education institutions...

     (UK).

Related indices


Some related values, also calculated and published by the same organization, are:
  • the immediacy index
    Immediacy index
    An immediacy index is a measure of how topical and urgent work published in a scientific journal is. Along with the better known impact factor measure, it is a calculated each year by the Institute for Scientific Information for those journals which it indexes; both impact factors and immediacy...

    : 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 the H-index
H-index
The h-index is an index that attempts to measure both the productivity and impact of the published work of a scientist or scholar. The index is based on the set of the scientist's most cited papers and the number of citations that they have received in other publications...

). The relative number of citations an individual article receives is better viewed as citation impact
Citation impact
Citation is the process of acknowledging or citing the author, year, title, and locus of publication of a source used in a published work. Such citations can be counted as measures of the usage and impact of the cited work. This is called citation analysis or bibliometrics...

.

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. 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 large, but controversial, influence on the way published scientific research is perceived and evaluated.

PageRank algorithm


In 1976 a recursive impact factor that gives citations from journals with high impact greater weight than citations from low-impact journals was proposed.
Such a recursive impact factor resembles the PageRank
PageRank
PageRank is a link analysis algorithm, named after Larry Page and used by the Google Internet search engine, that assigns a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web, with the purpose of "measuring" its relative importance within the set...

 algorithm of the Google
Google
Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...

 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
Herbert van de Sompel
Herbert Van de Sompel is a Belgian librarian and computer scientist, most known for his role in the development of the Open Archives Initiative and standards such as OpenURL, Object Reuse and Exchange, and the OAI Protocol for Metadata Harvesting....

 also proposed using the PageRank
PageRank
PageRank is a link analysis algorithm, named after Larry Page and used by the Google Internet search engine, that assigns a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web, with the purpose of "measuring" its relative importance within the set...

 algorithm. From their paper:
| ISI Impact Factor | PageRank | Combined
1 52.28 ANNU REV IMMUNOL  16.78 Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

 
51.97 Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

2 37.65 ANNU REV BIOCHEM  16.39 Journal of Biological Chemistry
Journal of Biological Chemistry
The Journal of Biological Chemistry is a weekly peer-reviewed scientific journal that was established in 1905. Since 1925 it is published by the American Society for Biochemistry and Molecular Biology. It covers research in any area of biochemistry or molecular biology. The editor-in-chief is...

 
48.78 Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

3 36.83 PHYSIOL REV
Physiological Reviews
Physiological Reviews is a journal published quarterly by the American Physiological Society. Physiological Reviews provides state of the art coverage of timely issues in the physiological and biomedical sciences. It appeals to physiologists, neuroscientists, cell biologists, biophysicists, and...

 
16.38 Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

 
19.84 New England Journal of Medicine
4 35.04 NAT REV MOL CELL BIO
Nature Reviews Molecular Cell Biology
Nature Reviews Molecular Cell Biology is a leading monthly review journal published by Nature Publishing Group. As its title suggests, it covers a broad range of topics within two distinct disciplines: molecular biology, the study of endogenous macromolecules, and cell biology, the structures,...

 
14.49 PNAS
Proceedings of the National Academy of Sciences
The Proceedings of the National Academy of Sciences of the United States of America, usually referred to as PNAS, is the official journal of the United States National Academy of Sciences...

 
15.34 Cell
Cell (journal)
Cell is a peer-reviewed scientific journal publishing research papers across a broad range of disciplines within the life sciences. Areas covered include molecular biology, cell biology, systems biology, stem cells, developmental biology, genetics and genomics, proteomics, cancer research,...

5 34.83 New England Journal of Medicine  8.41 PHYS REV LETT
Physical Review Letters
Physical Review Letters , established in 1958, is a peer reviewed, scientific journal that is published 52 times per year by the American Physical Society...

 
14.88 PNAS
Proceedings of the National Academy of Sciences
The Proceedings of the National Academy of Sciences of the United States of America, usually referred to as PNAS, is the official journal of the United States National Academy of Sciences...

6 30.98 Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

 
5.76 Cell
Cell (journal)
Cell is a peer-reviewed scientific journal publishing research papers across a broad range of disciplines within the life sciences. Areas covered include molecular biology, cell biology, systems biology, stem cells, developmental biology, genetics and genomics, proteomics, cancer research,...

 
10.62 Journal of Biological Chemistry
Journal of Biological Chemistry
The Journal of Biological Chemistry is a weekly peer-reviewed scientific journal that was established in 1905. Since 1925 it is published by the American Society for Biochemistry and Molecular Biology. It covers research in any area of biochemistry or molecular biology. The editor-in-chief is...

7 30.55 Nature Medicine
Nature Medicine
Nature Medicine is an academic journal publishing research articles, reviews, news and commentaries in the biomedical area, including both basic research and early-phase clinical research. Topics covered include cancer, cardiovascular disease, gene therapy, immunology, vaccines, and neuroscience...

 
5.70 New England Journal of Medicine  8.49 JAMA
Journal of the American Medical Association
The Journal of the American Medical Association is a weekly, peer-reviewed, medical journal, published by the American Medical Association. Beginning in July 2011, the editor in chief will be Howard C. Bauchner, vice chairman of pediatrics at Boston University’s School of Medicine, replacing ...

8 29.78 Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

 
4.67 Journal of the American Chemical Society
Journal of the American Chemical Society
The Journal of the American Chemical Society is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the Journal of Analytical and Applied Chemistry and the American Chemical Journal...

 
7.78 The Lancet
The Lancet
The Lancet is a weekly peer-reviewed general medical journal. It is one of the world's best known, oldest, and most respected general medical journals...

9 28.18 NAT IMMUNOL
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

 
4.46 J IMMUNOL
Journal of Immunology
The Journal of Immunology is a peer-reviewed academic journal that publishes basic and clinical studies in all aspects of immunology. It was founded by Arthur Fernandez Coca in 1915 . It is the official journal of The American Association of Immunologists and published by The American Association...

 
7.56 NAT GENET
Nature Genetics
Nature Genetics is a scientific journal concerning genetics. It is published by Nature Publishing Group, and was founded as part of the Nature family of journal in 1992. The 2010 impact factor is 36.377. Its sister journal is Nature Reviews Genetics.- External links :*...

10 28.17 REV MOD PHYS
Reviews of Modern Physics
The Reviews of Modern Physics is a journal of the American Physical Society. The journal started in paper form. All volumes are also online by subscription.Issue 1, Volume 1 consisted of the review by...

 
4.28 APPL PHYS LETT
Applied Physics Letters
Applied Physics Letters is a peer-reviewed scientific journal that is published 52 times per year by the American Institute of Physics. Its focus is rapid publication and dissemination of new experimental and theoretical papers regarding applications of physics in all disciplines of science,...

 
6.53 Nature Medicine
Nature Medicine
Nature Medicine is an academic journal publishing research articles, reviews, news and commentaries in the biomedical area, including both basic research and early-phase clinical research. Topics covered include cancer, cardiovascular disease, gene therapy, immunology, vaccines, and neuroscience...



The table shows the top 10 journals by ISI
Institute for Scientific Information
The Institute for Scientific Information was founded by Eugene Garfield in 1960. It was acquired by Thomson Scientific & Healthcare in 1992, became known as Thomson ISI and now is part of the Healthcare & Science business of the multi-billion dollar Thomson Reuters Corporation.ISI offered...

 Impact Factor, PageRank, and a modified system that combines the two (based on 2003 data). Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...

and Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

are generally regarded as the most prestigious journals, and in the combined system they come out on top.

The Eigenfactor
Eigenfactor
The Eigenfactor score, developed by Jevin West and Carl Bergstrom at the University of Washington, is a rating of the total importance of a scientific journal...

 is another PageRank
PageRank
PageRank is a link analysis algorithm, named after Larry Page and used by the Google Internet search engine, that assigns a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web, with the purpose of "measuring" its relative importance within the set...

-type measure of journal influence, with rankings freely available online.

Article level metrics


Starting in March 2009, the Public Library of Science
Public Library of Science
The Public Library of Science is a nonprofit open-access scientific publishing project aimed at creating a library of open access journals and other scientific literature under an open content license...

 introduced "article level metrics" on every article in all of their titles.

See also

  • h-index
    H-index
    The h-index is an index that attempts to measure both the productivity and impact of the published work of a scientist or scholar. The index is based on the set of the scientist's most cited papers and the number of citations that they have received in other publications...

    , for the impact factor of individual scientists, rather than journals.
  • PageRank
    PageRank
    PageRank is a link analysis algorithm, named after Larry Page and used by the Google Internet search engine, that assigns a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web, with the purpose of "measuring" its relative importance within the set...

    , the algorithm used by Google
    Google
    Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...

    , based on similar principles.
  • Eigenfactor
    Eigenfactor
    The Eigenfactor score, developed by Jevin West and Carl Bergstrom at the University of Washington, is a rating of the total importance of a scientific journal...

    , another journal citation ranking method.
  • SCImago Journal Rank
    SCImago Journal Rank
    SCImago Journal Rank is a measure of scientific influence of scholarly journals that accounts for both the number of citations received by a journal and the importance or prestige of the journals where such citations come from. The SJR indicator is a variant of the eigenvector centrality measure...

    , an open access journal metric which is based on Scopus data and uses an algorithm similar to PageRank.
  • Lowry protein assay
    Lowry protein assay
    The Lowry protein assay is a biochemical assay for determining the total level of protein in a solution. The total protein concentration is exhibited by a color change of the sample solution in proportion to protein concentration, which can then be measured using colorimetric techniques. It is...

     paper by Oliver Lowry
    Oliver Lowry
    Oliver Howe Lowry was an American biochemist. He is best remembered for devising the Lowry protein assay.Lowry was the youngest of a family of five children. His father was a teacher and later an administrator in the Chicago public school system...

     - one of the most cited papers in the scientific literature (cited over 200,000 times).

External links
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