The Journal Impact Factor and the 5-Year Impact Factor are generally considered as indicators of journal quality and status or prestige. These journal metrics are a measure of the frequency with which the “average article” in the journal has been cited in a given year.
The standard Impact Factor goes back to JCR 1997. To calculate this metric a two-year citation window is used. Its value is calculated by dividing the number of citations received in the given year to all items (all article document types) published in the journal in the two previous years by the number of citable items (articles, reviews and proceedings papers) published in the journal in the two previous years. An Impact Factor of 1.5 in a given year means that, on average, the items (all article document types) published in the journal one or two year ago have been cited one and a half time in the given year.
The 5-Year Impact Factor goes back to JCR 2007. This performance metric has been added to JCR, because a citation window of only two years is too short in many subject fields. In calculating the 5-Year Impact Factor a five-year citation window is used. A 5-year Impact Factor of 1.5 in a given year means that, on average, the items (all article document types) published in the journal in the five previous years have been cited one and a half time in the given year.
You can compare and evaluate (rank) all journals within a subject category by Impact Factors and other performance metrics. The Journal Summary List Table displays the journals that match your selection and sort criteria. The Impact Factor Trend Graph shows a journal’s impact over a five-year period.
Impact Factor Trend Graph of Agricultural Systems in 2010 JCR Science Edition
Journal Impact Factors should be used with caution in evaluating journal quality and status. If interested, read more about the limitations of Journal Impact Factors in this commentary: The ups and downs of journal impact factors. by Ogden and Bartley (2008).