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The impact factor (IF) of an academic journal is a measure reflecting the 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, with journals with higher impact factors 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 those journals that are indexed in the ''Journal Citation Reports''. ==Calculation== In any 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.〔(【引用サイトリンク】title=Introducing the Impact Factor )〕 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: : 2008 impact factor = ''A''/''B''. where: : ''A'' = the number of times that all items published in that journal in 2006 and 2007 were cited by indexed publications during 2008. : ''B'' = the total number of "citable items" published by that journal in 2006 and 2007. ("Citable items" for this calculation are usually articles, reviews, proceedings, or notes; not editorials or letters to the editor). (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. Occasionally, Thomson Reuters assigns an impact factor to new journals with less than two years of indexing, based on partial citation data.〔blogs.rsc.org. (), ''RSC Advances receives its first partial impact factor'', June 24, 2013. Retrieved on May 21st 2015.〕〔news.cell.com. (), ''Our first (partial) impact factor and our continuing (full) story'', July 30th, 2014. Retrieved on May 21st 2015.〕 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 five-year impact factor.〔(【引用サイトリンク】title=JCR with Eigenfactor )〕 The JCR shows rankings of journals by impact factor, if desired by discipline, such as organic chemistry or psychiatry. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Impact factor」の詳細全文を読む スポンサード リンク
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