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Byline: Tracey Caldwell

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The beguiling simplicity of the "impact factor" has made it a figure of supreme importance in research. Journal impact factors, or IFs, measure how often science and social science journals are cited by academics. The measurement of the number of times a journal is cited by researchers in the field has become shorthand for the value of that journal; and funding bodies and employers use citation metrics to assess the productivity of institutions, departments and individuals.

Thomson Scientific dominates the citation metrics landscape with its Web of Science-based citation index. Recently, however, it has faced increasing competition from the likes of Scopus and Google Scholar. The existence of realistic alternatives to Thomson Scientific's index A- and which give different results to it A- has thrown the debate on citation metrics wide open.

Thomson Scientific calculates the impact factor of a journal by dividing the number of current year citations into the number of source items published in that journal during the previous two years.