Journal Impact factor

The Impact factor, often abbreviated IF, is a measure of the citations to science and social science journals. It is frequently used as a proxy for the importance of a journal to its field.

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 average citation number of an article in that year.
* the journal 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 50% of all the citations from that journal in 2005.
* 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.

Overview

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".[1] Impact factors have a huge, but controversial, influence on the way published scientific research is perceived and evaluated.


Calculation

The impact factor for a journal is calculated based on a three-year period, and can be considered to be the average number of times published papers are cited up to two years after publication. For example, the 2003 impact factor for 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 articles, reviews, proceedings or notes 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 equation 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; while it is appropriate for some fields of science such as molecular biology, it is not for such 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. 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.

Small list of top 30 Journals according to IF



Full list for 2005 available here:

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Here are the 2006 ....top ten journals based on the impact factors among all science journals
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