The impact factor for journals was developed by Thomson Reuters in the late 1970’s, and they are published in their Journal Citation Reports. We have online access to this from 2004 to the present. (Older years are available for SCI and SSCI.) The 2011 report will probably be ready mid-2012.
This publication provides people with a sense of how often articles in a particular journal are cited. The more the articles in a specific journal are cited, then people will consider that journal to be more important. But, it should be noted that the impact factor number will go up and down each year, and that a couple of highly cited articles can skew the numbers for a couple of years. There are many criticisms of the impact factor, but it is still widely used as an indication of quality for journals.
I would recommend that you determine how a particular journal compares to others in a specific subject area. A journal with an impact factor of 3 in organic chemistry will be middle of the road, while another journal with an impact factor of 3 in geography would make it a top tier journal. Please use data from the Journal Citation Reports wisely.
There are other companies that publish journal metrics, such as:
If you have any questions about using the database, please let us know.