Bibliometrics has come of age and is generally accepted, together with peer review, as one way to describe the activity of players in science. Appealing issues in bibliometrics include the characterization of a growth regime of research systems including the trade-offs of production-visibility; the measure of science-technology linkage and coevolution; the measure of the societal effects of science; new models of collaboration applied to networks dynamics; and the multicriterion evaluation of actors, especially universities. In an article by Anthony F. J. van Raan titled "Measurement of Central Aspects of Scientific Research: Performance, Interdisciplinarity, Structure," van Raan describes a variety of state-of-the-art indicators that show the concern of bibliometricians for improving measures used in sensitive contexts such as research assessment. The sections of the article (performance, interdisciplinarity, structure) deal in various ways with the central question of the diversity of science as either an object or a framework for performance indicators. In this article, the author discusses a few practical issues stemming from this diversity. The author discusses how to define and structure a scientific field, whether normalization is straightforward, and which measure can be used for measuring knowledge flows and relations among fields. (Contains 4 footnotes.)