Current usability evaluation methods are essentially holistic in nature. However, engineers that apply a component-based software engineering approach might also be interested in understanding the usability of individual parts of an interactive system. This paper examines the efficiency dimension of usability by describing a method, which engineers can use to test, empirically and objectively, the physical interaction effort to operate components in a single device. The method looks at low-level events, such as button clicks, and attributes the physical effort associated with these interaction events to individual components in the system. This forms the basis for engineers to prioritise their improvement effort. The paper discusses face validity, content validity, criterion validity, and construct validity of the method. The discussion is set within the context of four usability tests, in which 40 users participated to evaluate the efficiency of four different versions of a mobile phone. The results of the study show that the method can provide a valid estimation of the physical interaction event effort users made when interacting with a specific part of a device.

The Cognitive Dimensions framework has inspired research both more and less varied than expected. In this paper we revisit the original aims and briefly describe some subsequent research, to consider whether the original aims were too austere in rejecting knowledge-based dimensions; whether the dimensions can be shown to have real-world relevance; and whether their definitions can be improved, either piecemeal or by refactoring the entire set. We mention some issues that remain unexplored, and conclude by describing two different ventures into defining clear procedures for real-life application, operating in very different milieux but both accepting that the framework should be developed from its original formulation. 2 1.

There is a wealth of user-centred evaluation methods (UEMs) to support the analyst in assessing interactive systems. Many of these support detailed aspects of use – for example: Is the feedback helpful? Are labels appropriate? Is the task structure optimal? Few UEMs encourage the analyst to step back and consider how well a system supports users ’ conceptual understandings and system utility. In this paper, we present CASSM, a method which focuses on the quality of ‘fit’ between users and an interactive system. We describe the methodology of conducting a CASSM analysis and illustrate the approach with three contrasting worked examples (a robotic arm, a digital library system and a drawing tool) that demonstrate different depths of analysis. We show how CASSM can help identify re-design possibilities to improve system utility. CASSM complements established evaluation methods by focusing on conceptual structures rather than procedures. Prototype tool support for completing a CASSM analysis is provided by Cassata, an open source development.