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.