ETAG, A Formal Model of Competence Knowledge for User Interface Design

This book is about a particular solution to the problem of how to design usable computer systems. Apart from this chapter, it may be read as a short history of the research efforts with respect to ETAG, which is the main subject of the book as well as the tool that is proposed to help create usable computer systems. Since answers are meaningless without the questions they address, this chapter discusses the questions underlying this research and, perhaps even more important, the reasons for asking these particular questions: the theoretical context which gave rise to them. This is especially important since a generally accepted approach to study and design in Human-Computer Interaction (HCI) is lacking. Moreover, the field might rather be characterised as a loosely organised collection of competing and often incompatible approaches. First, the chapter provides a general account of the subject matter of cognitive ergonomics, stressing the 'cognitive' part of it, and distinguishing it from related fields of research. Thereafter, the chapter focuses on creating usable computer systems as the primary aim of cognitive ergonomics, and it explains why this is not an easy task. Finally, the chapter defines cognitive ergonomics more precisely, as a science and engineer ing trade, that is concerned with user inter faces, and more specifically, their design. Guided by these three main characteristics of cognitive ergonomics, the major questions put forward in this thesis are derived and their background is discussed. 1.1 Cognitive Ergonomics Cognitive ergonomics is the study of human behaviour that is mediated by cognitive tools and devices. Cognitive tools are natural or artificial tools which require and determine the human ability to process information. The purpose of cognitive ergonomics is to adapt such cognitive tools and their usage so as to improve human information processing in terms of improved efficiency, fewer errors and accidents, and increased well-being. To this end it is necessary that cognitive ergonomics is able to analyse how such tools are used, to synthesise recommendations for design, and to evaluate uses and recommendations. An important area in cognitive ergonomics is the study of Human-Computer Interaction. Cognitive ergonomics is based on the premise that human interaction with computer devices is essentially a matter of knowledge representation and information processing, or: cognitive behaviour. Interacting with a computer system takes place by means of physical interaction: pressing buttons on a keyboard changes the physical state of the computer system which is fed back to 2 Chapter 1: Cognitive Ergonomics and UI Design the user by means of intensity changes of the light on the display unit. This description may be accurate, but it is as irrelevant as describing driving a car in terms of opening valves and pulling cables. It would rather be more sensible to describe interacting with a computer system in terms of writing a book with a word processor or calculating turnovers using a spreadsheet. To acquire 'common' human goals like writing a thesis or calculating the day's turnovers by means of a computer (or, for that matter any other tool, like a typewriter, pencil and paper, etc.), it is necessary to recursively subdivide and translate goals into commands for the word processor or spreadsheet. Since computer systems are not too sophisticated with respect to supporting human goals, the reformulation and translation processes of human goals and the interpretation of results of command invocations are necessarily human cognitive tasks. In cognitive ergonomics, interacting with a computer is assumed to involve different stages of human behaviour, such as formulating an intention and executing an action, and different levels of activity, such as the intention to improve a text and the intention behind physically executing a particular command (Norman, 1984). In cognitive ergonomic terms, users have to apply their knowledge about the computer system, in the form of a mental representation, to find the difference between a current state of affairs and a goal state within the user's task domain. Once established, the user has to devise a plan to diminish the difference, and reformulate the plan into the commands and command arguments of the computer system used. After issuing the commands, again, the user has to apply the knowledge about the system to transform feedback data from the computer into meaningful information about the success or failure of reaching the goal state within the task domain. As an example, consider making a paper copy of a report. In a paper office, one would acquire the report from its file, bring it to the copy machine to make a copy, and try not to forget to store the archive copy back in its place. In a 'paperless' office, it is necessary to know that reports are stored as computer files and that, in order to make a paper copy of a file, it has to be sent or copied to a printer device. The instruction how to copy a file varies between computer systems: dragging an icon representation of the report to a printer icon, typing in the name of the print command and the filename, etc. When the printer is not within visual or auditory reach, additional command specifications may be necessary to acquire information about the progress of the print command. Since computer usage primarily involves acquiring, transforming and applying (human) knowledge it may be clear that the basic thesis or central premise of cognitive ergonomics states that investigations should focus on cognitive factors in order to improve HumanComputer Interaction. The focus on cognitive factors distinguishes cognitive ergonomics from traditional or 'classical' ergonomics. As the name suggests, cognitive ergonomics may be seen as a mere branch of ergonomics, with which it shares the goal of facilitating human performance through adaptation of the tools to human characteristics and preferences. On the other hand, cognitive ergonomics differs from 'classical' ergonomics in that the focus is not so much on externally measurable quantities, such as movements, forces and body measures, but on A Formal Model for User Interface Design 3 psychological phenomena, such as knowledge, perception, and planning; phenomena that, generally, allow for indirect measurement only. Cognitive ergonomics is also closely related to cognitive psychology, in that both investigate a mental phenomena. Whereas cognitive psychology often uses computers to study human mental phenomena for the sake of acquiring general theories about mental behaviour, cognitive ergonomics studies mental phenomena and applies theoretical knowledge in order to solve the practical problems related to using computers. The abstract nature of cognitive psychological knowledge generally precludes applying it to practical problems without first making additional assumptions that may undermine the validity of solutions. Cognitive ergonomic theories are more directly connected to the domain of application. Finally, cognitive ergonomics is related to computer science in that both study the use of computer systems, but, whereas computer science investigates the technical requirements for using computers, cognitive ergonomics studies the human and cognitive requirements for doing so. Whereas cognitive ergonomics is related to general ergonomics and cognitive psychology via the subject matter of the investigations, computer science and cognitive ergonomics may be seen as mutual clients, where cognitive ergonomics is responsible for the design of the user interface (the user machine) that sets cognitive constraints, and computer science is responsible for the design and creation of the software (the soft machine) which sets and implements technical constraints for the overall design of computer systems. The fact that computer science and cognitive ergonomics are clients of each other does not express anything about their relative importance. Although there is a growing awareness that computer system design should include usability aspects, at present, especially in software engineering practice, the technical constraints are still predominant. Cognitive ergonomics and ergonomics in general are still regarded as additional to technical programming skills, rather than the opposite. Later on, it will be argued that, from a human task performance point of view, a far more important, and perhaps a leading role, should be assigned to cognitive ergonomics. At the start of this section, cognitive ergonomics was defined as the study to improve the use of cognitive tools in terms of efficiency, errors and accidents, and well-being. It may be possible to be more precise about the field and purpose of cognitive ergonomics. For example, humanitarians might insist that well-being refers to all human beings involved and exclude weapons, technocrats might want to focus on work systems and exclude enabling tools, and rationalists might want to restrict attention to purposive tools and exclude toys and pleasure. These statements may be caricatures but they do exemplify the risk of losing generality by playing the language game to strictly. Despite that cognitive ergonomics is not served by a priori excluding application areas and a more precise definition will not be provided, the work that is discussed in this thesis is restricted to the purposive usage of tools by means of discrete task performance. Computer systems are most commonly used to support work systems which makes it an obvious choice in HCI to focus on users who perform tasks for the purpose of acquiring specific goals and, because of the way to issue commands to computer systems, to focus on performing discrete tasks. 4 Chapter 1: Cognitive Ergonomics and UI Design Discrete tasks put specific requirements on cognitive processes such as perception, memory and attention which may differ considerably from the requirements of th