An alternative to field-normalization in the aggregation of heterogeneous scientific fields

A possible solution to the problem of aggregating heterogeneous fields in the all-sciences case relies on the normalization of the raw citations received by all publications. In this paper, we study an alternative solution that does not require any citation normalization. Provided one uses sizeand scale-independent indicators, the citation impact of any research unit can be calculated as the average (weighted by the publication output) of the citation impact that the unit achieves in all fields. The two alternatives are confronted when the research output of the 500 universities in the 2013 edition of the CWTS Leiden Ranking is evaluated using two citation impact indicators with very different properties. We use a large Web of Science dataset consisting of 3.6 million articles published in the 2005-2008 period, and a classification system distinguishing between 5,119 clusters. The main two findings are as follows. Firstly, differences in production and citation practices between the 3,332 clusters with more than 250 publications account for 22.5% of the overall citation inequality. After the standard field-normalization procedure where cluster mean citations are used as normalization factors, this figure is reduced to 4.3%. Secondly, the differences between the university rankings according to the two solutions for the all-sciences aggregation problem are of a small order of magnitude for both citation impact indicators.