Display-Based Competence: Towards User Models for Menu-Driven Interfaces

This paper discusses the critical role played by aspects of the display in the use of many computer systems especially those driven by menus. We outline a formal model of “display-based competence” by extending the Task-Action Grammar notation ( Payne & Green, 1986 ). The model, D-TAG (for display-orinted task-action grammar) is illustrated with examples for the well-known Macintosh desk-top interface, and from a more deeply-nested menu interface to a device used for the remote testing of telephone line (RATES). D-TAG exploits two extensions of TAG to address important aspects of interface consistency. The most important extension uses a featural description of the display to capture the role of the display in structuring task-action mappings; the second describes the “side-effects” of a task, i.e. those effects not described by the semantic attributes of a task. By embedding these extensions within the organizing framework of TAG's feature-grammar, we are able to develop descriptions of interfaces which highlight aspects of (display) design that are outside the scope of other formal user models.