The Impact factor, often abbreviated IF, is a measure of the citations to science and social science journals. A citation is a reference to a source (not always the original source published or unpublished(citation needed For a broader class of publications which include scientific journals see Academic journal. It is frequently used as a proxy for the importance of a journal to its field. 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
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. Eugene "Gene" Garfield (born September 16 1925 in New York City) is an American Scientist, one of the founders of Bibliometrics The Institute for Scientific Information (ISI was founded by Eugene Garfield in 1960. This article is primarily about Thomson prior to its 2008 merger with Reuters 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. Journal Citation Reports ( JCR) is an Annual publication by the Institute of Scientific Information, a division of Thomson Scientific Some related values, also calculated and published by the same organization, are:
These measures apply only to journals, not individual articles or individual scientists (unlike, say, the H-index). The h-index is an index that quantifies both the actual scientific productivity and the apparent scientific impact of a scientist The relative number of citations an individual article receives is better viewed as citation impact. Citation is the process of acknowledging or citing the author year title and locus of publication (journal book or other of a source used in a published work
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 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. Organic chemistry is a discipline within Chemistry which involves the scientific study of the structure properties composition reactions, and preparation Psychiatry is a medical specialty which exists to study, prevent, and treat Mental disorders in Humans Psychiatric
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 quality. 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:
The most commonly mentioned faults of the impact factor include:
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. Nature is a prominent Scientific journal, first published on 4 November 1869 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. The normal distribution, also called the Gaussian distribution, is an important family of Continuous probability distributions applicable in many fields It is rather a Bradford distribution, as predicted by theory. 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 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). The University of California Los Angeles (generally known as UCLA) is a public research university located in Westwood Los Angeles, California, United 
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.
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. PageRank is a link analysis algorithm that assigns a numerical weighting to each element of a Hyperlinked set of documents such as the World Wide Web, Google Inc is an American public corporation, earning revenue from advertising related to its Internet search, e-mail, online 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. PageRank is a link analysis algorithm that assigns a numerical weighting to each element of a Hyperlinked set of documents such as the World Wide Web,  From their paper:
|ISI Impact Factor||PageRank||Combined|
|1||52. 28||ANNU REV IMMUNOL||16. This is the article on the publisher Annual Reviews whose titles are invariably called Annual review of. 78||Nature||51. Nature is a prominent Scientific journal, first published on 4 November 1869 97||Nature|
|2||37. Nature is a prominent Scientific journal, first published on 4 November 1869 65||ANNU REV BIOCHEM||16. This is the article on the publisher Annual Reviews whose titles are invariably called Annual review of. 39||Journal of Biological Chemistry||48. The Journal of Biological Chemistry (often abbreviated JBC) is a Scientific journal founded in 1905 and published since 1925 by the American Society 78||Science|
|3||36. Science is the Academic journal of the American Association for the Advancement of Science and is considered one of the world's most prestigious Scientific 83||PHYSIOL REV||16. 38||Science||19. Science is the Academic journal of the American Association for the Advancement of Science and is considered one of the world's most prestigious Scientific 84||New England Journal of Medicine|
|4||35. The New England Journal of Medicine ( N Engl J Med or NEJM) is an English-language Peer-reviewed Medical journal published 04||NAT REV MOL CELL BIO||14. Nature Reviews Molecular Cell Biology (ISSN 1471-0080 is a leading monthly review journal ( Impact Factor for 2004 of 33 49||PNAS||15. 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 34||Cell|
|5||34. Cell is a biweekly peer-reviewed scientific journal which publishes novel research in any area of experimental biology that is significant outside its field 83||New England Journal of Medicine||8. The New England Journal of Medicine ( N Engl J Med or NEJM) is an English-language Peer-reviewed Medical journal published 41||PHYS REV LETT||14. Physical Review Letters is one of the most prestigious journals in Physics. 88||PNAS|
|6||30. 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 98||Nature||5. Nature is a prominent Scientific journal, first published on 4 November 1869 76||Cell||10. Cell is a biweekly peer-reviewed scientific journal which publishes novel research in any area of experimental biology that is significant outside its field 62||Journal of Biological Chemistry|
|7||30. The Journal of Biological Chemistry (often abbreviated JBC) is a Scientific journal founded in 1905 and published since 1925 by the American Society 55||Nature Medicine||5. Nature Medicine is an Academic journal publishing research articles reviews news and commentaries in the biomedical area including both basic research and 70||New England Journal of Medicine||8. The New England Journal of Medicine ( N Engl J Med or NEJM) is an English-language Peer-reviewed Medical journal published 49||JAMA|
|8||29. JAMA The Journal of the American Medical Association is an international peer-reviewed general Medical journal, published 48 times per year by the American 78||Science||4. Science is the Academic journal of the American Association for the Advancement of Science and is considered one of the world's most prestigious Scientific 67||Journal of the American Chemical Society||7. The Journal of the American Chemical Society (usually abbreviated as J 78||The Lancet|
|9||28. This article is about the journal For other uses of the term "lancet" see Lancet (disambiguation. 18||NAT IMMUNOL||4. Nature is a prominent Scientific journal, first published on 4 November 1869 46||J IMMUNOL||7. The Journal of Immunology ( The JI) is an Academic journal that publishes basic and clinical studies in all aspects of Immunology. 56||NAT GENET|
|10||28. Nature is a prominent Scientific journal, first published on 4 November 1869 17||REV MOD PHYS||4. The Reviews of Modern Physics is a journal of the American Physical Society. 28||APPL PHYS LETT||6. Applied Physics Letters is a weekly Peer-reviewed Scientific journal published by the American Institute of Physics devoted to the publication of new experimental 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 Medicine is an Academic journal publishing research articles reviews news and commentaries in the biomedical area including both basic research and The Institute for Scientific Information (ISI was founded by Eugene Garfield in 1960. Nature and Science are generally regarded as the most prestigious journals, and in the combined system they come out on top. Nature is a prominent Scientific journal, first published on 4 November 1869 Science is the Academic journal of the American Association for the Advancement of Science and is considered one of the world's most prestigious Scientific 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. The New England Journal of Medicine ( N Engl J Med or NEJM) is an English-language Peer-reviewed Medical journal published It is necessary to analyze the data for a journal in the light of a detailed knowledge of the journal literature.
The Eigenfactor is another PageRank-type measure of journal influence, with rankings freely available at eigenfactor.org. PageRank is a link analysis algorithm that assigns a numerical weighting to each element of a Hyperlinked set of documents such as the World Wide Web,