How Can Journal Impact Factors be Normalized across Fields of Science?

Loet Leydesdorff loet at LEYDESDORFF.NET
Fri May 4 01:56:16 EDT 2012

How Can Journal Impact Factors be Normalized across Fields of Science? An
Assessment in terms of Percentile Ranks and Fractional Counts

Loet Leydesdorff, Ping Zhou, & Lutz Bornmann


Using the CD-ROM version of the Science Citation Index 2010 (N = 3,705
journals), we study the (combined) effects of (i) fractional counting on the
impact factor (IF) and (ii) transformation of the skewed citation
distributions into a distribution of 100 percentiles and six percentile rank
classes (top-1%, top-5%, etc.). Do these approaches lead to field-normalized
impact measures for journals? In addition to the two-year IF (IF2), we
consider the five-year IF (IF5), the respective numerators of these IFs, and
the number of Total Cites, counted both as integers and fractionally. These
various indicators are tested against the hypothesis that the classification
of journals into 11 broad fields by PatentBoard/National Science Foundation
provides (statistically) significant between-field effects. Using fractional
counting the between-field variance is reduced by 91.7% in the case of IF5,
and by 79.2% in the case of IF2. However, the differences in citation counts
are not significantly affected by fractional counting. These results accord
with previous studies, but the longer citation window of a fractionally
counted IF5 can lead to significant improvement in the normalization across


Preprint version available at 


** apologies for cross-postings



Loet Leydesdorff 

Amsterdam School of Communications Research (ASCoR)
Kloveniersburgwal 48, 1012 CX Amsterdam.
Tel. +31-20-525 6598; fax: +31-842239111

 <mailto:loet at> loet at ;
Visiting Professor,  <> ISTIC,
Beijing; Honorary Fellow,  <> SPRU, University
of Sussex;  <>  


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