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Again : Journal Impact Factor October 5, 2012

Posted by harisvanjava in Serbaneka.

The Journal Impact Factor is published each year by Thomson Reuters. It measures the number of times an average paper in a particular journal has been referred to.

The Impact Factor of journal J in the calendar year X is the number of citations received by J in X to any item published in J in (X-1) or (X-2), divided by the number of source items published in J in (X-1) or (X-2).

Source items

‘Source items’ is the term used to refer to full papers: original research articles, reviews, full length proceedings papers, rapid or short communications, and so on. Non-source items, such as editorials, short meeting abstracts, and errata, are not counted in the denominator although any citations they might receive will be included in the numerator.

An example follows for the fictitious Journal of Great Science:

* In year X, the Journal of Great Science received 152 citations to items published in (X-1) and 183 citations to items published in (X-2). Total citations for Impact Factor calculation = 335.

* 123 source items were published in the Journal of Great Science in (X-1), and 108 in (X-2). Total source items for Impact Factor calculation = 231.

* Year X Impact Factor for the Journal of Great Science = 335/231 = 1.450.

Impact Factor can be affected by subject field, number of authors, content type, and the size of the journal; this is described in our Perspectives in Publishing paper, from which the figure above, showing a generalized citation curve and how Thomson Reuters’ metrics relate to it, is taken.

The Impact Factor can be a useful way of comparing citability of journals, if the comparison is limited to a given subject field and the type of journals being compared (review, original research, letters) are similar. The absolute Impact Factor is of limited use, without that of other journals in the field against which to judge it.

You can find the most recent Impact Factors of our individual journals on their homepages.

Five-year Impact Factor

The five-year Impact Factor is similar in nature to the regular ‘two-year’ Impact Factor, but instead of counting citations in a given year to the previous two years and dividing by source items in these years, citations are counted in a given year to the previous five years and again divided by the source items published in the previous five years.

An example for Tetrahedron Letters:
2-yr Impact Factor: 9621 citations in 2010 to items published in 2008 and 2009 / 3675 items published in 2008 and 2009 = 2.618
5-yr Impact Factor: 23846 citations in 2010 to items published in 2005, 2006, 2007, 2008, and 2009 / 9602 items published in 2005-2009 = 2.483

A base of five years may be more appropriate for journals in certain fields because the body of citations may not be large enough to make reasonable comparisons or it may take longer than two years to disseminate and respond to published works. The two measures differ also in the amount of variability between years. The two-year Impact Factor can fluctuate by around 20% in value each year, whereas the five-year measure, while still showing changes over time, presents a much smoother variation.

The exact number in the metric may differ, but often this difference disappears when one looks at the relative position of a journal within its subject field. If the whole field evolves slower and benefits from a 5-yr measure, the rankings will not differ much.


Journals are often ranked by Impact Factor in an appropriate Thomson Reuters subject category. As there are now two published Impact Factors, this rank may be different when using a two- or a five-year Impact Factor and care is needed when assessing these ranked lists to understand which metric is being utilized. In addition, journals can be categorized in multiple subject categories which will cause their rank to be different and consequently a rank should always be in context to the subject category being utilized.

*)Source :




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