How fractional counting affects the Impact Factor: Steps towards
field-independent classifications of scholarly journals and literature 


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 .

Loet Leydesdorff
University of Amsterdam

Lutz Bornmann
ETH Zurich

** apologies for cross-postings