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

The usual solution to the aggregation problem of heterogeneous fields in the all-sciences case relies onthe prior normalization of the raw citations received by any publication. In this paper, we propose analternative solution that does not require any prior field-normalization. The citation impact of anyresearch unit can be calculated as the average (weighted by the publication output) of the citation impact that the unit achieves in all fields. Using a large WoS dataset consisting of 3.6 million publications in the 2005-2008 period, and an algorithmically constructed publication-level classification system that distinguishes between 5,119 clusters, this simple alternative is confrontedwith the usual one 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. The main two findings are as follows. Firstly, differences in production and citation practices between 3,332 clusters greater than 250 publications account for 22.5% of the overall citation inequality. Afterthe standard field-normalization procedure where cluster mean citations are used as normalization factors, this figure is reduced to 4.3%. Secondly, in this scenario, the differences between the university rankings according to the two solutions to the all-sciences aggregation problem when the original 5,119 clusters are considered, are of a small order of magnitude for both citation impact indicators.