Abstract
In this paper we present an experiment and two replications aimed at comparing the support provided by ER and UML class diagrams during comprehension activities by focusing on the single building blocks of the two notations. This kind of analysis can be used to identify weakness in a notation and/or justify the need of preferring ER or UML for data modeling. The results reveal that UML class diagrams are generally more comprehensible than ER diagrams, even if the former has some weaknesses related to three building blocks, i.e., multi-value attribute, composite attribute, and weak entity. These findings suggest that a UML class diagram extension should be considered to overcome these weaknesses and improve the comprehensibility of the notation.