  Usability is a key quality attribute of successful software systems.
Unfortunately, there is no common understanding of the factors influencing
usability and their interrelations. Hence, the lack of a comprehensive basis
for designing, analyzing, and improving user interfaces. This paper proposes a
2-dimensional model of usability that associates system properties with the
activities carried out by the user. By separating activities and properties,
sound quality criteria can be identified, thus facilitating statements
concerning their interdependencies. This model is based on a tested quality
meta-model that fosters preciseness and completeness. A case study demonstrates
the manner by which such a model aids in revealing contradictions and omissions
in existing usability standards. Furthermore, the model serves as a central and
structured knowledge base for the entire quality assurance process, e.g. the
automatic generation of guideline documents.
