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Everything you always wanted to know about Extended Task-Action Grammar but were afraid to ask.

ETAG: Geert de Haan


What is ETAG?

Extended Task-Action Grammar (ETAG) is a formalism proposed by Michael Tauber (and developed by the uppersigned) to represent user- interface designs. More specific, ETAG represent the knowledge which a perfectly knowing user has about a particular user interface. It does not specify how users represent knowledge; merely what they should know about both the interaction language (the knowledge "how-to") and about the interactive device (the knowledge "how it works").
ETAG is a formal model, and can thus be used for user interface analysis, even before a single line of code has been written. As a formal representation of a user interface; including the knowledge a user needs about the application, it can be used as a source for automatic user documentation and on-line help generation. In the same way, it has been used as a (though rudimentary) system prototyping tool. Finally, since ETAG describes user knowledge about user interfaces, it is an especially suitable basis for a user interface design method: sufficiently formal to bridge the 'gap' with software engineering, while sufficiently exact to serve as a communication means, and sufficiently user-oriented to ensure usable computer systems. What is not sufficient -as yet- is the usability of the ETAG notation for designers; something that will be addressed through the use of design tools that hide the peculiarities of the notation.

What kind of Formal Model is ETAG?

For those familiar with formal modelling in HCI: ETAG is Task-Action Grammar (TAG, Green and Payne) with a device model build on top of it. It is like Command Language Grammar (CLG, Moran) but without the ad-hoccery and its redundancy. In comparison with Cognitive Complexity Theory (CCT, Kieras and Polson) and GOMS (Goals, etc.; Card, Moran and Newell) it is a pure competence model instead of a performance model.
In comparison to the work of the York group, Petri-nets and the like, ETAG is not concerned with software engineering characteristics of a user interface, but with user-knowledge, building upon an ontology of the 'virtual machinery' of the interface (Jackendoff and Sowa) and employing conceptual graphs.
In terms of design methods based on formal representations, ETAG-based design comes closest to Muse*/JSD (Long and Lim) and Adept (Johnson and Wilson). Through the use of JSD, Muse*/JSD may be easier to connect to software Engineering design, and Adept seems more advanced in terms of the tools it provides, but ETAG-based design is based on a superior knowledge representation scheme.

What does an ETAG representation look like?

This is an extremely tiny example; only to get the 'taste' of ETAG's formalism. It shows the type definition of a Clipboard and how it is used in a Cut_String event as a temporal storage space. The Cut_String event is invoked by a user task with (accidentally) the same name.
Type definition:

  type [OBJECT = CLIPBOARD]
    supertype: [SINGLE_OBJECT_BOARD] ;
    themes:    [STRING: *s] | [WORD: *w] | [PICTURE: *p] | ... ;
    instances: [CLIPBOARD: #clipboard] ;
  end  [CLIPBOARD]

  type [EVENT = CUT_STRING]
    description:   event.MOVE_TO [STRING: *s, OBJECT: #clipboard] ;
    precondition:  state.IS_AT [STRING: *s, OBJECT: #document] ;
                   state.HAS_VAL [STRING: *s, ATTRIBUTE: selected] ;
    clears:        state.IS_AT [STRING: *s, OBJECT: #document] ;
    postcondition: state.IS_AT [STRING: *s, OBJECT: #clipboard] ;
                   state.HAS_VAL [STRING: *s, ATTRIBUTE: purgable] ;
    comments: "move a string from a document on to the clipboard"
  end [CUT_STRING].

Basic tasks:

  Entry: 2
    task:     CUT_STRING
    event:    CUT_STRING
    system:   PLACE = clipboard, PLACE = document
    T1 [EVENT = CUT_STRING, OBJECT = STRING: *s, ATTRIBUTE = selected]
    comment: "move a selected string from a document to the clip"

What's written about ETAG?

The publications listed below are available as html files. The papers are listed in more-or-less chronological order and each deals with a particular aspect of ETAG. The last entry, PhD thesis, gives the best overview of ETAG, in terms of background and motivation, content and application, the design method and conclusions about ETAG.

Formal Representation in Human-Computer Interaction
Geert de Haan

This paper discusses the reasons to use formal models of user knowledge in human-computer interaction. Four classes of formal models are identified: models for task environment analysis, models to analyze user knowledge, user performance prediction models, and representation models for design purposes.
The main problems mentioned in the literature as regards the use of these models will be discussed. Finally, the contribution of our research group to the NFI-project "Systematic Design of User Interfaces" is presented, which is directed at the Extended Task-Action Grammar (ETAG, Tauber, 1988, 1990). This includes applying ETAG to the presentation component and the semantics of the user interface, providing tools to increase ETAG's usability for designers, and performing validation studies.

Appeared in: van der Veer, G.C., White, T.N. and Arnold, A.G. (1993, eds.) Human-Computer Interaction: Proc. Preparing for the Nineties, 95-112. Stichting Informatica Congressen, Amsterdam. ISBN 90-5005-039-5.

Formal Modelling Techniques in Human-Computer Interaction
G. de Haan, G.C. van der Veer & J.C. van Vliet

This paper is a theoretical contribution, elaborating the concept of models as used in Cognitive Ergonomics. A number of formal modelling techniques in human-computer interaction will be reviewed and discussed. The analysis focuses on different related concepts of formal modelling techniques in human-computer interaction. The label "model" is used in various ways to represent the knowledge users needs to operate interactive computer systems, to represent user relevant aspects in the design of interactive systems, and to refer to methods that generate evaluative and predictive statements about usability aspects of such systems.
The reasons underlying the use of formal models is discussed and a review is presented of the most important modelling approaches, which include External- Internal Task Mapping Analysis; Action Language; Task-Action Grammar; the Goals, Operators, Methods and Selection model; Command Language Grammar and Extended Task-Action Grammar. The problems associated with applying the present formal modelling techniques are reviewed, and possibilities to solve these problems are presented. Finally, we conclude with a discussion of the future work that needs to be done, i.e., the development of a general design approach for usable systems, and the need to focus attention on the practice of applying formal modelling techniques in design.

Appeared in: Acta Psychologica, 78, nos. 1-3, 26-76. Also appeared in: Van der Veer, G.C., Bagnara, S. and Kempen, G.A.M. (1992), (eds.), Cognitive Ergonomics - contributions from experimental psychology. North-Holland, Amsterdam, pp. 27-68.

Extended Task-Action Grammar (ETAG): the Psychological Basis of a Formal Model for User Interface Design
Geert de Haan

In this paper we discuss the psychological basis of Extended Task-Action Grammar (ETAG). ETAG is both a method to represent the knowledge a competent user of a user interface has about the interface and a method to represent the user interface for the purpose of design. The psychological consideration underlying the format of the model are: the use of a feature grammar to represent the interaction language, and the distinction of the description into four levels of abstraction, similar to those in (natural) language processing.
The psychological choices underlying the content of the model consider the choice of the canonical basis. This is the most abstract level of representation in ETAG. The concepts defined at this level are most general and necessary to acquire any knowledge of the world, and which serve as the source from which all other concepts in ETAG are derived. The concepts in the canonical basis are derived from psychological research into semantic memory by Klix and colleagues, and from psycho-linguistic analyses into the semantics which underlie language by Sowa and Jackendoff.
The paper concludes with an example of an ETAG representation and a brief comparison between ETAG and other design models in Human-Computer Interaction.

Unpublished manuascript (1995).

A case study on applying Extended Task-Action Grammar (ETAG) to the design of a human-computer interface.
G. de Haan & N. Muradin

In this paper Extended Task Action Grammar (ETAG; Tauber, 1988, 1990) is applied, as a formal method to represent the knowledge a competent user has about the structure of a user interface. This paper consists of three parts.
First, ETAG itself will be discussed, including a brief treatment of the considerations underlying its development and the purposes of ETAG, an explanation of what an ETAG description consists of, and some remarks about the similarities and differences between ETAG and other formal models.
Secondly, the models are represented which resulted from applying ETAG to the design problems formulated in the Macinter modelling contest (MacInter 1990), supplemented with remarks about specific problems and choices taken. This part will be directed at both evaluating the design alternatives of the contest and a preliminary evaluation of ETAG as a design model.
Finally, using the results of the previous section, the general problems, possibilities and shortcomings that were encountered in applying ETAG will be discussed in an attempt to formulate requirements for the future development of the method.

Appeared in: Zeitschrift für Psychology, 200, 1992, 135-156.

Analyzing user interfaces: ETAG validation studies
Geert de Haan & Gerrit C. van der Veer

In this paper we will discuss the use of Extended Task-Action Grammar (ETAG) as a tool to analyze user interfaces. An ETAG representation is a formal model of the knowledge of a competent user about how the computer system works, and about how to perform tasks.
The structure of ETAG representations is briefly explained, and methods of applying ETAG analyses are discussed. Several studies are reviewed in which ETAG is applied to different types of computer systems, and to variants of the same system, in order to reach conclusions about the validity and the general applicability of ETAG, and to determine which aspects of the method may be improved, and how this may be done.

Appeared in: Proc. of the 13th interdisciplinary workshop on Informatics and Psychology - Task-Analysis in Human-Computer Interaction, Schärding, Austria, 9-11 June, 1992.

Etag as the Basis for Intelligent Help Systems
G. de Haan & G.C. van der Veer

ETAG is a method for representing interactive computer systems for design purposes, on the basis of a representation of what a competent (perfectly knowing) user knows about the structure and use of a system. An ETAG representation is a conceptual model which contains all information a user might want to have of a computer system. As such, ETAG representations may serve as the basis for intelligent help facilities. A number of studies will be reviewed in which ETAG was used to build non- interactive and interactive facilities providing help information about computer systems.
In order to provide static -non changing- information about the computer system, the ETAG representations contain sufficient information. To provide dynamic information about interacting with the system as well, the information in ETAG representations needs to be supplemented with history information.
In order to cope with dynamically changing information, it proved to be necessary and more widely advantageous to use an intermediate (Prolog) representation of ETAG and history information. In different respects, following a modular system architecture proved to be advantageous.

Appeared in: van der Veer, G.C., Tauber, M.J. and Bagnara, S. (eds.). Proc. ECCE-6. Human-Computer Interaction: Tasks and Organisation, 271- 283. CUD, Rome, Italy, 1992. ISBN 88-7721-232-2.

An ETAG-based Approach to the Design of User-interfaces
Geert de Haan

This paper describes ETAG-based design, as an approach to design user interface on the basis of a formal model of the knowledge, a competent user needs to use the interface to perform tasks with it. The approach is based on the (obvious) idea that user interfaces are for people to use to perform tasks. ETAG-based design is a user-centered, task-centered and (conceptual) knowledge-based method to design user interfaces. In design, these principles are embodied within the standard model of design (the received view), by using the ETAG formalism to specify a conceptual model of the user's task world: the Virtual Machine.
First, the received view of user interface design is discussed, and it is concluded that this view lacks guarantees that user interfaces are designed for people to use.
Second, ETAG is briefly presented. ETAG is a formal model to specify the user interface from the point of view of a user's competence knowledge. Thirdly, we discuss ETAG-based design and show how it is used within the received view of user interface design, to kindly persuade designers to take a user (task) oriented stance.
Finally, we draw a few conclusions regarding the near- and far future of ETAG-based design, or more in general, user-model-based design.

Appeared in: Proceedings of the 15th interdisciplinary workshop on "Informatics and Psychology" - Interdisciplinary approaches to system analysis and design, Schärding, Austria, 24-26 May, 1994.

Note: the material listed above was presented in a handout entitled: "All you ever wanted to know about Extended Task-Action Grammar but were afraid to ask" at the Seventh European Conference on Cognitive Ergonomics, ECCE-7, September 5-8, 1994, Bonn, Germany. The material listed below was added to this document later on.

How to Cook ETAG and Related Dishes: Uses of a Notational Language for User Knowledge Representation for User Interface Design.
Geert de Haan

Extended Task-Action Grammar (ETAG) is a modelling notation to formally describe user interfaces in terms of the knowledge, a competent user would have in order to use the interface for task performance. After introducing the most important ideas underlying ETAG and, briefly, its place in the historical context of formal modelling in HCI, the structure, contents and how to create ETAG models are extensively discussed. Hereafter, we briefly mention some of the uses of the notation, and the structure of ETAG-based user interface design. After discussing the status of ETAG-based design in software engineering, several conclusions are drawn.

Appeared in: Cognitive Systems 4, 3-4, 353-379, 1997.

ETAG-based Design: User Interface Design as User Mental Model Design.
Geert de Haan

Extended Task-Action Grammar (ETAG) is a formal language to represent user interfaces in terms of the knowledge (the mental model) that a perfectly knowing user would have about performing tasks. In ETAG-based design, user interface design is regarded as the incremental specification of the mental model of a perfectly knowing user.
The design process is structured after the ETAG model into discrete steps, each covering a specific set of design decisions. ETAG-based design is an example of model-based design, and it shows what an HCI specific design, aiming at usable computer systems should look like.

Appeared in: Palanque, P. and Benyon, D. (1996)(eds.) Critical Issues in User-Interface System Engineering, Springer Verlag.

Accomodating Diverse Users in ETAG and ETAG-Based Design: task knowledge and presentation.
Geert de Haan

This paper discusses ETAG, a formal model for design representation, and ETAG-based design, a method for user interface design, and the two principal facilities they provide to accommodate different types of users in the design of user interfaces. The paper starts with a description of the purpose and the structure of ETAG. This is followed by a description of ETAG-based design and using the notation to represent relevant aspects of the work context. Thereafter, in relation to differences and similarities in required task knowledge, the two main facilities in ETAG-based design for addressing different types of users are discussed and exemplified. Finally, using evidence from several design projects, conclusions are drawn about addressing differences among users in ETAG and ETAG-based design.

Appeared in: Proceedings of HCI International 2001, 5-10 August 2001, New Orleans, USA.

ETAG, a Formal Model of Competence Knowledge for User Interface Design
Geert de Haan

Formal models for user-interface design are important because formality provides a number of advantages, such as early use in the design process, precise prediction of usability aspects, precision for design and communication, and enabling automatic generation of user interfaces.

Within the domain of formal models, the thesis attempts to answer two main questions:
(1) what is a good formal specification model for user-interface design?
(2) what is a good method for user-interface design, based on such a model?

To answer the first question, a number of criteria are developed for evaluating the available formal models such as ETIT, TAG, CLG, GOMS, and ETAG. Mainly on the basis of the completeness of the conceptual specification and the psychological validity, ETAG (Extended Task-Action Grammar) is selected for further development. Specifically aimed at validating the notation, the thesis discusses the psychological basis and it provides a manual for creating the representations.

The second question is answered from a radical user-centered perspective: the user interface is defined as everything a user must know in order to perform tasks, software design is treated as subordinate to user interface design. ETAG-based User Interface Design follows the common scheme for user interface design, except that ETAG is used as the main representation method, and iteration between design stages is avoided as much as possible. Aimed at validating both the notation and the design method, the thesis discusses task-analysis, user-interface analysis, and the generation of help information and user interface prototypes.

The main conclusions from the study are as follows. ETAG and ETAG-based User Interface Design perform very well. There are two main areas for further development. To address the relevance of perceptual information to task performance, methods and tools are required to describe and analyse the perceptual interface. To address the relevance of the dynamical aspects of the interaction to task performance, such as learning, it is necessary to bridge the gap between static knowledge models, like ETAG, and dynamic knowledge models, like SOAR.

Appeared as PhD thesis, Department of Mathematics and Computer Science, Free University Amsterdam, 10 October 2000.

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