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How fractional counting affects the Impact Factor: Steps towards field-independent classifications of scholarly journals and literature <http://www.leydesdorff.net/weighted_if/weighted_if.pdf> Abstract The ISI-Impact Factors suffer from a number of drawbacks, among them the statistics-why should one use the mean and not the median?-and the incomparability among fields of science because of systematic differences in citation behavior among fields. Can these drawbacks be counteracted by counting citation weights fractionally instead of using integers? (i) Fractional citation counts are normalized in terms of the citing papers and thus would take into account differences in citation behavior among fields of science. (ii) Differences in the resulting distributions can be tested statistically for their significance at different levels of aggregation. (iii) Fractional counting can be generalized to any document set including journals or groups of journals, and thus the significance of differences among both small and large sets can be tested. In addition to the Impact Factor, the Total Cites of the journals listed in the Science Citation Index (CD-Rom version) 2008 are analyzed in these terms. The between-group variances (among fields) are tested using a Poisson regression model. A list of fractionally counted Impact Factors and Total Cites for 2008 is available online at http://www.leydesdorff.net/weighted_if/weighted_if.xls . Loet Leydesdorff University of Amsterdam Lutz Bornmann ETH Zurich ** apologies for cross-postings