The Immediacy Index is the average number of times an article is cited in the year it is published. It indicates how quickly articles in a journal are cited. It is published annually by Thomson Reuters in Journal Citation Reports.
The immediacy index can provide a useful perspective for comparing journals specializing in cutting-edge research. Because it is a per-article average, the Immediacy Index tends to discount the advantage of large journals over small ones. However, frequently issued journals may have an advantage because an article published early in the year has a better chance of being cited than the one published later in the year. Many publications that publish infrequently or late in the year have low Immediacy Indexes.
The Immediacy Index is calculated by dividing the number of citations to articles published in a given year by the number of articles published in that year.
The calculation of Immediacy Index for Cell. Image source: Journal Citation Reports (Thomson Reuters)<http://www.isiknowledge.com/> 2.4.2009