How fractional counting affects the Impact Factor: Normalization in terms of differences in citation potentials among fields of science

TL;DR

This paper investigates how fractional counting of citations can normalize impact factors across different scientific fields by accounting for variations in citation behavior, leading to more comparable metrics.

Contribution

It introduces a fractional counting method for impact factors that normalizes citation counts based on citing sources, addressing field differences and enabling statistical testing.

Findings

Fractional counting reduces differences among fields in impact factors.

Normalized impact factors show no significant variance among fields.

Fractional counting can be applied to various document sets for comparison.

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 whole numbers in the numerators? (i) Fractional citation counts are normalized in terms of the citing sources 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. A list of fractionally counted Impact Factors for 2008 is available online at http://www.leydesdorff.net/weighted_if/weighted_if.xls. The in-between group variance among the thirteen fields of science identified in the U.S. Science and Engineering Indicators is not statistically significant after this normalization. Although citation behavior differs largely between disciplines, the reflection of these differences in fractionally counted citation distributions could not be used as a reliable instrument for the classification.