The impact factor, which is based on the citation indexes in the Web of Science and published by Thomson Corporation, is a measure of identifying the predominance or lack thereof of specific journals in particular disciplines. The impact factor is calculated by dividing the number of citations in one particular year to articles published in two previous years within one specific journal by the number of articles published in the same previous two years of the same journal. The impact factor of journals, whether open access or not, can be determined provided they are indexed in Web of Science, and can assist authors in deciding in what journal within their discipline to publish.
A study by McVeigh for Thomson Scientific in 2004 found that open access journals had competitive numbers in terms of impact factor ratings. Another study by Testa and McVeigh for Thomson Scientific in the same year found similar results.
It should be noted that the impact factor is a quantitative measure and cannot be equated with the quality of individual articles and that not every journal published is indexed in the Web of Science databases. Furthermore, studies have recently questioned the validity of data used to calculated the impact factor since the number of citations refer to a different set of articles as the number of articles to which the citation compare (see Rossner, M., Van Epps, H. and Hill, E. "Show me the data ", Journal of Cell Biology (2007): 179: 1091-1092 http://www.jcb.org/cgi/content/full/179/6/1091).