How can journal impact factors be normalized across fields of science? An assessment in terms of percentile ranks and fractional counts

  • Authors:

  • Loet Leydesdorff;Ping Zhou;Lutz Bornmann

  • Affiliations:

  • Amsterdam School of Communications Research (ASCoR), University of Amsterdam, Kloveniersburgwal 48, 1012 CX, Amsterdam, The Netherlands;Institute of Scientific and Technical Information of China, 15th Fuxing Road, Haidian District, Beijing, China;Max Planck Society, Division for Science and Innovation Studies, Hofgartenstraße 8, 80539, Munich, Germany

  • Venue:
  • Journal of the American Society for Information Science and Technology
  • Year:
  • 2013

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Abstract

Using the CD-ROM version of the Science Citation Index 2010 (N = 3,705 journals), we study the (combined) effects of (a) fractional counting on the impact factor (IF) and (b) 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 2-year IF (IF2), we consider the 5-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/NSF (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 fields. © 2013 Wiley Periodicals, Inc.