The impact factor (IF) or journal impact factor (JIF) of an academic journal is a measure reflecting the yearly average number of citations to recent articles published in that journal. It is frequently used as a proxy for the relative importance of a journal within its field; journals with higher impact factors are often deemed to be more important than those with lower ones. The impact factor was devised by Eugene Garfield, the founder of the Institute for Scientific Information. Impact factors are calculated yearly starting from 1975 for journals listed in the Journal Citation Reports.
In any given year, the impact factor of a journal is the number of citations, received in that year, of articles published in that journal during the two preceding years, divided by the total number of articles published in that journal during the two preceding years:
This means that, on average, its papers published in 2012 and 2013 received roughly 41 citations each in 2014. Note that 2014 impact factors are reported in 2015; they cannot be calculated until all of the 2014 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. Occasionally, Journal Citation Reports assigns an impact factor to new journals with less than two years of indexing, based on partial citation data. The calculation always uses two complete and known years of item counts, but for new titles one of the known counts is zero. 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. For example, the Journal Citation Reports (JCR) also includes a five-year impact factor, which is calculated by dividing the number of citations to the journal in a given year by the number of articles published in that journal in the previous five years.
Journal impact factors are often used to evaluate the merit of individual articles and individual researchers. This particular use of impact factors was summarised by Hoeffel:
Impact Factor is not a perfect tool to measure the quality of articles but there is nothing better and it has the advantage of already being in existence and is, therefore, a good technique for scientific evaluation. Experience has shown that in each specialty the best journals are those in which it is most difficult to have an article accepted, and these are the journals that have a high impact factor. Most of these journals existed long before the impact factor was devised. The use of impact factor as a measure of quality is widespread because it fits well with the opinion we have in each field of the best journals in our specialty.
In conclusion, prestigious journals publish papers of high level. Therefore, their impact factor is high, and not the contrary.
As impact factors is a journal-level metric, rather than an article or individual level metric, this use is controversial. Garfield agrees with Hoeffel, but warns about the "misuse in evaluating individuals" because there is "a wide variation [of citations] from article to article within a single journal".
Some companies are producing false impact factors. According to an article published in the United States National Library of Medicine, these include Global Impact Factor (GIF), Citefactor, and Universal Impact Factor (UIF).
Numerous criticisms have been made regarding the use of impact factors. For one thing, the impact factor might not be consistently reproduced in an independent audit. There is also a more general debate on the validity of the impact factor as a measure of journal importance and the effect of policies that editors may adopt to boost their impact factor (perhaps to the detriment of readers and writers). Other criticism focuses on the effect of the impact factor on behavior of scholars, editors and other stakeholders. Others have made more general criticisms, arguing that emphasis on impact factor results from negative influence of neoliberal policies on academia claiming that what is needed is not just replacement of the impact factor with more sophisticated metrics for science publications but also discussion on the social value of research assessment and the growing precariousness of scientific careers in higher education.
It has been stated that impact factors and citation analysis in general are affected by field-dependent factors which may invalidate comparisons not only across disciplines but even within different fields of research of one discipline. The percentage of total citations occurring in the first two years after publication also varies highly among disciplines from 1–3% in the mathematical and physical sciences to 5–8% in the biological sciences. Thus impact factors cannot be used to compare journals across disciplines.
Because citation counts have highly skewed distributions, the mean number of citations is potentially misleading if used to gauge the typical impact of articles in the journal rather than the overall impact of the journal itself. For example, about 90% of Nature's 2004 impact factor was based on only a quarter of its publications, and thus the actual number of citations for a single article in the journal is in most cases much lower than the mean number of citations across articles. Furthermore, the strength of the relationship between impact factors of journals and the citation rates of the papers therein has been steadily decreasing since articles began to be available digitally.
Indeed, impact factors are sometimes used to evaluate not only the journals but the papers therein, thereby devaluing papers in certain subjects. The Higher Education Funding Council for England was urged by the House of Commons Science and Technology Select Committee to remind Research Assessment Exercise 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. The effect of outliers can be seen in the case of the article "A short history of SHELX", which included this sentence: "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 (at 31.434) and Science (at 28.103). The second-most cited article in Acta Crystallographica Section A in 2008 only had 28 citations. Also, impact factor is a journal metric and should not be used to assess individual researchers or institutions.
A.E. Cawkell, sometime Director of Research at the Institute for Scientific Information remarked that the Science Citation Index (SCI), on which the impact factor is based, "would work perfectly if every author meticulously cited only the earlier work related to his theme; if it covered every scientific journal published anywhere in the world; and if it were free from economic constraints."
A journal can adopt editorial policies to increase its impact factor. For example, journals may publish a larger percentage of review articles which generally are cited more than research reports. Thus 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. Some journal editors set their submissions policy to "by invitation only" to invite exclusively senior scientists to publish "citable" papers to increase the journal impact factor.
Journals may also attempt to limit the number of "citable items"—i.e., the denominator of the impact factor equation—either by declining to publish articles that are unlikely to be cited (such as case reports in medical journals) or by altering articles (e.g., by not allowing an abstract or bibliography in hopes that Journal Citation Reports will not deem it a "citable item"). As a result of negotiations over whether items are "citable", impact factor variations of more than 300% have been observed. Items considered to be uncitable—and thus are not incorporated in impact factor calculations—can, if cited, still enter into the numerator part of the equation despite the ease with which such citations could be excluded. This effect is hard to evaluate, for the distinction between editorial comment and short original articles is not always obvious. For example, letters to the editor may refer to either class.
Another less insidious tactic journals employ is to publish a large portion of its papers, or at least the papers expected to be highly cited, early in the calendar year. This gives those papers more time to gather citations. 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.
Beyond editorial policies that may skew the impact factor, journals can take overt steps to game the system. For example, in 2007, the specialist journal Folia Phoniatrica et Logopaedica, 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 scientific situation in some countries" related to 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.
Coercive citation is a practice in which an editor forces an author to add extraneous citations to an article before the journal will agree to publish it, in order to inflate the journal's impact factor. A survey published in 2012 indicates that coercive citation has been experienced by one in five researchers working in economics, sociology, psychology, and multiple business disciplines, and it is more common in business and in journals with a lower impact factor. However, cases of coercive citation have occasionally been reported for other disciplines.
Because "the impact factor is not always a reliable instrument", in November 2007 the European Association of Science Editors (EASE) 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 (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 many possible solutions—e.g., considering a limit number of publications per year to be taken into consideration for each scientist, or even penalising scientists for an excessive number of publications per year—e.g., more than 20.
In February 2010, the Deutsche Forschungsgemeinschaft (German Research Foundation) 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 and the impact factor". This decision follows similar ones of the National Science Foundation (US) and the Research Assessment Exercise (UK).
In response to growing concerns over the inappropriate use of journal impact factors in evaluating scientific outputs and scientists themselves, the American Society for Cell Biology together with a group of editors and publishers of scholarly journals created the San Francisco Declaration on Research Assessment (DORA). Released in May 2013, DORA has garnered support from thousands of individuals and hundreds of institutions, including in March 2015 the League of European Research Universities (a consortium of 21 of the most renowned research universities in Europe), who have endorsed the document on the DORA website.
Some related values, also calculated and published by the same organization, include:
Additional journal-level metrics are available from other organizations. For example, CiteScore: is a metric for serial titles in Scopus launched in December 2016 by Elsevier. While these metrics apply only to journals, there are also author-level metrics, such as the H-index, that apply to individual researchers. In addition, article-level metrics measure impact at an article level instead of journal level. Other more general alternative metrics, or "altmetrics", may include article views, downloads, or mentions in social media.
Fake impact factors are produced by companies not affiliated with Journal Citation Reports. These are often used by predatory publishers; Jeffrey Beall maintained a list of such misleading metrics. Consulting Journal Citation Reports' master journal list can confirm if a publication is indexed by Journal Citation Reports, which is a necessary (but not sufficient) condition for obtaining an IF. Use of fake impact metrics is considered a "red flag".
a measure of the speed at which content in a particular journal is picked up and referred to
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 American Journal of Botany is a monthly peer-reviewed scientific journal which covers all aspects of plant biology. It has been published by the Botanical Society of America since 1914. According to the Journal Citation Reports, the journal has a 2012 impact factor of 2.586. Access is available through JSTOR with a moving wall of 5 years.American Journal of Mathematics
The American Journal of Mathematics is a bimonthly mathematics journal published by the Johns Hopkins University Press.Animal Cognition
Animal Cognition is a peer-reviewed scientific journal published by Springer Science+Business Media. It covers research in ethology, behavioral ecology, animal behavior, cognitive sciences, and all aspects of human and animal cognition. According to the Journal Citation Reports, the journal has a 2017 impact factor of 2.805.Australian Systematic Botany
Australian Systematic Botany is an international peer-reviewed scientific journal published by CSIRO Publishing. It is devoted to publishing original research, and sometimes review articles, on topics related to systematic botany, such as biogeography, taxonomy and evolution. The journal is broad in scope, covering all plant, algal and fungal groups, including fossils.
First published in 1978 as Brunonia, the journal adopted its current name in 1988.
The current Editor-in-Chief is Daniel Murphy (Royal Botanic Gardens Melbourne).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, immunology, neuroscience, structural biology, microbiology, virology, physiology, biophysics, and computational biology. The journal was established in 1974 by Benjamin Lewin and is published twice monthly by Cell Press, an imprint of Elsevier.Gene (journal)
Gene is a peer-reviewed scientific journal in genetics and molecular biology, focusing on the cloning, structure, function, as well as the biomedical and biotechnological importance of genes. It was established in 1976 and is published by Elsevier.According to the Journal Citation Reports, the journal has a 2015 impact factor of 2.319.. The 2017 impact factor of the journal is 2.498.
Beyond Gene, there are several sub-specialty journals linked to Gene including Meta Gene, Plant Gene, Agri Gene and Gene Reports.Geology (journal)
Geology is a peer-reviewed publication of the Geological Society of America (GSA). GSA claims that it is the most widely read scientific journal in the field of earth science. It is published monthly, with each issue containing 20 or more articles. In 2017, the journal's impact factor was 4.635.One of the goals of the journal is to provide a forum for shorter articles (four pages each) and less focus on purely academic research–type articles.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 areas of biochemistry and molecular biology. The editor-in-chief is Lila Gierasch. All its articles are available free after one year of publication. In press articles are available free on its website immediately after acceptance.Journal of Ornithology
The Journal of Ornithology (formerly Journal für Ornithologie) is a scientific journal published by Springer Science+Business Media on behalf of the Deutsche Ornithologen-Gesellschaft. It was founded by Jean Cabanis in 1853, becoming the official journal of the Deutsche Ornithologen-Gesellschaft in 1854.
The first issue was produced in January 1853 and Cabanis noted that although there were specialist journals in entomology and conchology that there was nothing to deal with ornithology in Germany. Among the first essays published in the journal was an essay by Reichenbach on the concept of species.According to the Journal Citation Reports, the journal has a 2012 impact factor of 1.632.Journal of Vertebrate Paleontology
The Journal of Vertebrate Paleontology is a bimonthly peer-reviewed scientific journal that was established in 1980 by Jiri Zidek (University of Oklahoma). It covers all aspects of vertebrate paleontology, including vertebrate origins, evolution, functional morphology, taxonomy, biostratigraphy, paleoecology, paleobiogeography, and paleoanthropology. The journal is published by Taylor & Francis on behalf of the Society of Vertebrate Paleontology. According to the Journal Citation Reports, the journal has a 2017 impact factor of 2.190.Malacologia
Malacologia is a peer-reviewed scientific journal in the field of malacology, the study of mollusks. The journal publishes articles in the fields of molluscan systematics, ecology, population ecology, genetics, molecular genetics, evolution, and phylogenetics.The journal specializes in publishing long papers and monographs. The journal publishes at least one, sometimes two, volumes of about 400 pages per year, which may consist of 1 or 2 issues. According to the Journal Citation Reports, its 2010 impact factor is 1.024. This ranks Malacologia 66th out of 145 listed journals in the category "Zoology". The journal started publication in 1962.Nature Genetics
Nature Genetics is a scientific journal founded as part of the Nature family of journals in 1992. It publishes high quality research in genetics.
The journal encompasses genetic and functional genomic studies on human traits and on other model organisms, including mouse, fly, nematode and yeast. Current emphasis is on the genetic basis for common and complex diseases and on the functional mechanism, architecture and evolution of gene networks, studied by experimental perturbation.
It is an internationally recognized scientific publication with an Impact Factor of 27.959, making it the second ranked journal in the category of genetics and heredity, second to Nature Reviews Genetics, so first in research. Its sister journal is Nature Reviews Genetics.Palaeontology (journal)
Palaeontology is one of the two scientific journals of the Palaeontological Association (the other being Papers in Palaeontology). It was established in 1957 and is published on behalf of the Association by Wiley-Blackwell. The editor-in-chief is Andrew Smith (Natural History Museum, London). Palaeontology publishes articles on a range of palaeontological topics, including taphonomy, functional morphology, systematics, palaeo-environmental reconstruction and biostratigraphy. According to the Journal Citation Reports, the journal has a 2017 impact factor of 3.730, ranking it 1st out of 55 journals in the category "Paleontology".Physical Review Letters
Physical Review Letters (PRL), established in 1958, is a peer-reviewed, scientific journal that is published 52 times per year by the American Physical Society. As also confirmed by various measurement standards, which include the Journal Citation Reports impact factor and the journal h-index proposed by Google Scholar, many physicists and other scientists consider Physical Review Letters to be one of the most prestigious journals in the field of physics.PRL is published as a print journal, and is in electronic format, online and CD-ROM. Its focus is rapid dissemination of significant, or notable, results of fundamental research on all topics related to all fields of physics. This is accomplished by rapid publication of short reports, called "Letters". Papers are published and available electronically one article at a time. When published in such a manner, the paper is available to be cited by other work. The Lead Editor is Hugues Chaté. The Managing Editor is Reinhardt B. Schuhmann.Reviews of Modern Physics
Reviews of Modern Physics is a quarterly peer-reviewed scientific journal published by the American Physical Society. It was established in 1929 and the current editor-in-chief is Michael Thoennessen. The journal publishes review articles, usually by established researchers, on all aspects of physics and related fields. The reviews are usually accessible to non-specialists and serve as introductory material to graduate students, which survey recent work, discuss key problems to be solved and provide perspectives toward the end. RMP is arguably one of the most, if not the most, prestigious, authoritative and highly impacting journals in the field of physics. Its most recent impact factor for 2016 is 36.917 with a 5-year impact factor of 45.547.TESOL Quarterly
TESOL Quarterly is a quarterly peer-reviewed academic journal published by Wiley-Blackwell on behalf of TESOL International Association. It covers English language teaching and learning and standard English as a second dialect, including articles on the psychology and sociology of language learning and teaching, professional preparation, curriculum development, and testing and evaluation. The editors-in-chief are Charlene Polio and Peter De Costa, both at Michigan State University. TESOL also publishes TESOL Journal.
According to the Journal Citation Reports, the journal had a 2016 impact factor of 2.056, ranking it 14th out of 182 journals in the category "Linguistics" and 34th out of 235 journals in the category "Education & Educational Research". There has been a substantial increase in the past three years under the editorial leadership of previous editors, Brian Paltridge and Ahmar Mahboob, both of the University of Sydney: the 2015 impact factor was 1.513, and 2014 impact factor was 0.940.Tetrahedron (journal)
Tetrahedron is a weekly peer-reviewed scientific journal covering the field of organic chemistry. According to the Journal Citation Reports, the journal has a 2014 impact factor of 2.641. Tetrahedron and Elsevier, its publisher, support an annual symposium. In 2010, complaints were raised over its high subscription cost.Tetrahedron Letters
Tetrahedron Letters is a weekly international journal for rapid publication of full original research papers in the field of organic chemistry. According to the Journal Citation Reports, the journal has a 2014 impact factor of 2.379 and it is ranked 22nd out of 57 journals in the "Organic Chemistry" category.Webometrics
The science of webometrics (also cybermetrics) tries to measure the World Wide Web to get knowledge about the number and types of hyperlinks, structure of the World Wide Web and usage patterns. According to Björneborn and Ingwersen (2004), the definition of webometrics is "the study of the quantitative aspects of the construction and use of information resources, structures and technologies on the Web drawing on bibliometric and informetric approaches." The term webometrics was first coined by Almind and Ingwersen (1997). A second definition of webometrics has also been introduced, "the study of web-based content with primarily quantitative methods for social science research goals using techniques that are not specific to one field of study" (Thelwall, 2009), which emphasizes the development of applied methods for use in the wider social sciences. The purpose of this alternative definition was to help publicize appropriate methods outside of the information science discipline rather than to replace the original definition within information science.
Similar scientific fields are Bibliometrics, Informetrics, Scientometrics, Virtual ethnography, and Web mining.
One relatively straightforward measure is the "Web Impact Factor" (WIF) introduced by Ingwersen (1998). The WIF measure may be defined as the number of web pages in a web site receiving links from other web sites, divided by the number of web pages published in the site that are accessible to the crawler. However the use of WIF has been disregarded due to the mathematical artifacts derived from power law distributions of these variables. Other similar indicators using size of the institution instead of number of webpages have been proved more useful.
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