The influence of international collaboration on the impact of research results - Some simple mathematical considerations concerning the role of self-citations

There is an ongoing discussion on the influence of international collaboration on impact as measured by citation-based indicators. Collaboration generally involves more authors than 'no collaboration' work and it is obvious that the phenomenon of self-citation will be stronger (there are more authors to cite themselves). Thus it can be seen as an important amplifier' of measured impact. Although this effect is certainly possible and already demonstrated recently, it should not be considered as the only or even major explanation of higher impact in the comparison between 'no collaboration' and international collaboration. Using data of an extensive bibliometric study of astronomical research in the Netherlands, we prove that higher rates of self-citation in international collaboration do not play any significant role as 'impact amplifier'. The central point is that proper impact measurement must involve corrections for self-citations.