Architectural component models represent high level designs and are frequently used as a central view of architectural descriptions of software systems. They play a crucial role in the whole development process and in achieving the desired software qualities. This paper presents an empirical study that examines the impact of hierarchies on the architecture-level software understandability. In particular we have studied three different architectural representations of a large-size software system, one with a hierarchical representation where architectural components at all abstraction levels in the hierarchy are shown, and two that do not contain hierarchical abstractions but concentrate only on the lowest level or on the highest level components in the hierarchy. We conducted a controlled experiment in which participants of three groups received one of the three architecture documentations plus the source code of the system and had to answer understandability related questions. Our results show that using the hierarchical architecture leads to: 1) higher quantity of correctly retrieved elements, 2) lower quantity of incorrectly retrieved elements, and 3) higher overall quality of retrieved elements. The obtained results provide empirical evidence that hierarchies play an important role in the context of architectural component models from the viewpoint of the architecture-level software understandability.

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