  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.
