Clarivate Analytics (formerly Thomson Reuters Scientific) analyzes journal citation patterns to determine impact factors. The Journal Citation Reports (JCR) for Sciences and Social Sciences journals compile these data into ranked lists of journals. These values are used to measure the importance of individual journals relative to other titles in their field.
The UC San Diego Library's online subscription to the JCR covers 2007 forward. The Library also holds annual print/microfiche reports, with data from 1998 - 2007 / 2008.
Science ed. 1998 - 2007 Microforms, Geisel Floor2 West
Social Sciences ed. 1998 - 2008 Microforms, Geisel Floor 2 West