"The Impact Factor is a very useful tool for evaluation of journals, but it must be used discretely. Considerations include the amount of review or other types of material published in a journal, variations between disciplines, and item-by-item impact." Eugene Garfield
How scientific research is evaluated, recognized, and perceived affects the lives of scientists, the distribution of research funds, and the entire process of doing science. When Eugene Garfield, founder of the Institute for Scientific Information (ISI), created the Science Citation Index (SCI) in the 60s, he could not have imagined that it would be used for purposes it had never been designed for. There are many reasons why the SCI and the Journal Impact Factor (IF) derived from it attracted so much attention.
Authors of scientific articles wanted to know how often their works have been cited, because they are often evaluated, hired, promoted, and funded on the basis of their citation counts and the quality (the IF) of the journals, in which they have published their papers.
Funding agencies are placing an increased weight on IF of journals in which the grant applicants have published their articles.
Editors are trying to understand how IF can be influenced so that could increase their journal's ranking
Publishers of scientific journals are using IFs to conduct market research and promote their journals.
Librarians are often making their decisions about which journals to drop or add on the basis of their IFs.
The IFs of journals are published in Journal Citation Reports (JCR). The IF of a particular journal is the ratio between the number of times articles published in the journal were cited, divided by the number of citable articles* published in this journal during the same period of time. For example, the IF of a journal for the year 2010 is calculated as follows: ISI (now Thomson-Reuters) counts the number of citations made in 2010 to papers published in this journal during the previous two years (2008 and 2009) and divides this number by the number of citable articles published in the same two-year period. JCR for a particular year reports data from the previous year. For example, JCR for 2011 reports the IFs for the year 2010.
* The way the term "citable articles" is defined has caused a lot of criticism. When the IF is calculated, the numerator includes all citations to the journal, whereas the denominator, instead of also including all articles published in the journal, excludes editorials, letters, and comments. This has allowed editors to manipulate the IF of their journals by publishing review articles as editorials, which are usually often cited. So the citations to these reviews are counted in the nominator but are excluded from the denominator, which increases this ratio and thus - the IF.