Multiplicative and fractional strategies when journals are assigned to several sub-fields

In many datasets, articles are classified into sub-fields through the journals in which they have been published. The problem is that many journals are assigned to a single sub-field, but many others are assigned to several sub-fields. This paper discusses a multiplicative and a fractional strategy to deal with this situation, and introduces a normalization procedure in the multiplicative case that takes into account differences in mean citation rates across sub-fields. The empirical part studies different aspects of citation distributions under the two strategies, namely: (i) the number of articles, (ii) the mean citation rate, (iii) the broad shape of the distribution, (iv) the characterization in terms of size- and scale-invariant indicators of high- and low-impact, and (v) the presence of extreme distributions, or distributions that behave very differently from the rest. It is found that, in spite of large differences in the number of articles according to both strategies, the similarity of the citation characteristics of articles published in journals assigned to one or several sub-fields guarantees that choosing one of the two strategies may not lead to a radically different picture in practical applications. Nevertheless, the evaluation of citation excellence through a high-impact indicator may considerably differ depending on that choice.