Concept-based Analysis of Surface and Structural Misfits (CASSM) Tutorial notes.

Ann Blandford, I Connell, T Green
in Working/Discussion Paper, London, UK


Concept-based Analysis of Surface and Structural Misfits (CASSM) in a novel approach to usability analysis that focuses attention on misfits between user and system concepts. We believe that as an approach it has several desirable qualities: o It focuses on concepts rather than tasks or procedures. Consequently, it complements the majority of existing approaches to usability evaluation. In particular, it analyses conceptual misfits between user and system. o By intentionally supporting 'sketchy' analysis, CASSM avoids the 'death by detail' that plagues many evaluation techniques. CASSM analyses do not have to be complete or consistent to be useful - though of course a thorough analysis is likely to have these properties. Also, CASSM analyses are often quite succinct, compared to (for example) a Cognitive Walkthrough (Wharton et al, 1994), Heuristic Evaluation (Nielsen, 1994) or GOMS analysis (John & Kieras, 1996). o As a notation, it provides a 'bridge' between the core ideas underpinning work on mental models and design issues, and may thus make prior work on mental models more readily accessible to design practice. [This should be regarded as a hypothesis that has not yet been tested.] o The CASSM notation provides a relatively formal definition of many of Green's Cognitive Dimensions (see, for example, Green, 1989; Green & Petre, 1996; Blackwell & Green 2003). In this way, it further supports assessment of a system in terms of CDs. This is discussed in detail towards the end of this document. Although the name (CASSM: Concept-based Analysis of Surface and Structural Misfits) emphasises the importance of misfits, you should be aware that there are other kinds of user-system misfits that are outside the scope of CASSM; for example, inconsistencies in procedures for similar tasks would be picked up by other techniques but are not directly addressed within CASSM. CASSM focuses on conceptual structures.