CWTS crown indicator measures citation impact of a research group's publication oeuvre

The article "Caveats for the journal and field normalizations in the CWTS (`Leiden') evaluations of research performance", published by Tobias Opthof and Loet Leydesdorff (arXiv:1002.2769) deals with a subject as important as the application of so called field normalized indicators of citation impact in the assessment of research performance of individual researchers and research groups. Field normalization aims to account for differences in citation practices across scientific-scholarly subject fields. As the primary author of the papers presenting the "Leiden" indicators and of many reports and articles reporting on the outcomes of assessments actually using these measures, I comment on the 3 main issues addressed in the paper by Opthof and Leydesdorff.