  A knowledge organization system (KOS) is made up of concepts and semantic
relations between the concepts which represent a knowledge domain
terminologically. We distinguish between five approaches to KOSs:
nomenclatures, classification systems, thesauri, ontologies and, as a
borderline case of KOSs, folksonomies. The research question of this paper is:
How can we informetrically analyze the effectiveness of KOSs? Quantitative
informetric measures and indicators allow for the description, for comparative
analyses as well as for evaluation of KOSs and their quality. We describe the
state of the art of KOS evaluation. Most of the evaluation studies found in the
literature are about ontologies. We introduce measures of the structure of KOSs
(e.g., groundedness, tangledness, fan-out factor, or granularity) and
indicators of KOS quality (completeness, consistency, overlap, and use).
