Different scientific fields have different citation practices. Citation-based bibliometric indicators need to normalize for such differences between fields in order to allow for meaningful between-field comparisons of citation impact. Traditionally, normalization for field differences has usually been done based on a field classification system. In this approach, each publication belongs to one or more fields and the citation impact of a publication is calculated relative to the other publications in the same field. Recently, the idea of source normalization was introduced, which offers an alternative approach to normalize for field differences. In this approach, normalization is done by looking at the referencing behavior of citing publications or citing journals. In this paper, we provide an overview of a number of source normalization approaches and we empirically compare these approaches with a traditional normalization approach based on a field classification system. We also pay attention to the issue of the selection of the journals to be included in a normalization for field differences. Our analysis indicates a number of problems of the traditional classification-system-based normalization approach, suggesting that source normalization approaches may yield more accurate results.
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Differences in citation density between fields may also be caused by unidirectional citation flows between fields (e.g., from applied fields to more basic fields) and by differences between fields in the growth rate of the literature. The source normalization approach does not correct for these effects (e.g., Zitt and Small 2008).
WoS covers a substantial number of trade magazines. Examples of some of the larger ones are Genetic Engineering & Biotechnology News, Naval Architect, and Professional Engineering. Popular magazines covered by WoS include, among others, the scientific magazines American Scientist, New Scientist, and Scientific American and the business magazines Forbes and Fortune.
In the case of the Netherlands, WoS for instance covers the Dutch language journals Psychologie & Gezondheid, Tijdschrift voor Communicatiewetenschap, and Tijdschrift voor Diergeneeskunde as well as the English language journals Economist-Netherlands, Netherlands Heart Journal, and Netherlands Journal of Medicine.
In the case of a journal that is assigned to multiple fields in a field classification system, e i is calculated as the harmonic average of the expected numbers of citations obtained for the different fields. For a justification of this approach, we refer to Waltman et al. (2011).
In a somewhat different context, the idea of fractional citation counting was already suggested by Small and Sweeney (1985).
Counting all references in a citing publication instead of only active references disadvantages fields with a relatively large number of references to older publications and to publications in journals not covered by one’s database.
The full results of our analysis are available online at www.ludowaltman.nl/normalization/.
A similar conclusion is reached by Radicchi and Castellano (2012a). However, there is a fundamental difference between our analysis and the one by Radicchi and Castellano. Radicchi and Castellano apply fractional citation counting in the way it was originally proposed by Leydesdorff and Opthof (2010), which means that fractioning is done based on the total number of references in a citing publication. Instead of the total number of references, we look at the number of active references in a citing publication (cf. Leydesdorff et al. in press). Our analysis makes clear that taking into account only active references does not solve the problems of the fractional citation counting approach.
This problem is also discussed by Glänzel et al. (1999). As a solution, these authors propose to treat journals with a broad scope in a special way. In their proposal, publications in journals with a broad scope are assigned to fields based on their references.
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Recent studies on classification-system-based normalization approaches focus on identifying general patterns in the citation distributions of scientific fields (e.g., Crespo et al. 2012; Radicchi and Castellano 2012b; Radicchi et al. 2008). These studies usually do not exclude any journals. It seems likely that the results of these studies depend quite significantly on whether trade journals, popular magazines, and other special journals are included or excluded.
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We would like to thank Javier Ruiz Castillo for his comments on an earlier draft of this paper. We are also grateful to an anonymous referee for various useful comments.
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Waltman, L., van Eck, N.J. Source normalized indicators of citation impact: an overview of different approaches and an empirical comparison. Scientometrics 96, 699–716 (2013). https://doi.org/10.1007/s11192-012-0913-4
- Bibliometric indicator
- Citation analysis
- Field normalization
- Source normalization