VISUAL DISPLAY DESIGN: A COMPARISON OF TWO METHODOLOGIES Denis JAVAUXa, Marie-Isabelle COLARDb and Jean VANDERDONCKTc1 a Work Psychology, FAPSE Bat. B-32, Université de Liège, B-4000 Sart-Tilman, Belgium, Tel.: +32-(0)41/66.20.13, Fax : +32-(0)41/66.29.44, E-mail :
[email protected] b Tractebel Energy Engineering, av. Ariane, 7, B-1200 Brussels, Belgium, Tel.: +32-(0)2/773.70.17, E-mail :
[email protected] c Institut d’Informatique, Facultés Universitaires Notre-Dame de la Paix, rue Grandgagnage, 21, B-5000 Namur, Belgium, Tel. : +32-(0)81/72.49.75, Fax : +32-(0)81/72.49.67 Industrial Ergonomics, Design and Use of Information Systems, Process Control, Mimic Displays This paper summarizes two methodologies for visual display design belonging to two different activity domains. The first methodology can be used for designing mimic displays in process control situations. The role of mimic displays is to provide operators with the information required for achieving their task under every possible normal and abnormal circumstance. Mimic displays must not only inform operators on the status of the process subsystems, devices or variables but also on their structural and functional interrelations. The second methodology can be used for designing computer screens of business oriented interactive applications. Their role is to present to the user the information required to carry out an interactive task by the way of interaction objects. These objects are ergonomically suited to represent this information. A comparison of the two methodologies highlights some structural intersection in their underlying principles and steps. INTRODUCTION Designing visual displays for devices, monitors, control rooms and user interfaces of interactive applications is widely recognized to be highly iterative, hard to structure and difficult to make objective. Trying to make these visual displays as ergonomic as possible in a cognitive perspective, i.e. compatible with a related user task, increases these difficulties since industrial and software ergonomics remain today more an art than a science that can be applied straightforwardly. In order to overcome these shortcomings and to structure the work of ergonomic design, several methodologies have been introduced. Two of them will be presented here. The first methodology is devoted to mimic displays for dynamic process control. It relies on a contextual task analysis and graphical formalisms to represent task’s and information variables. It has been successfully applied by Tractebel Energy Engineering at the « Doel 3 » Belgian nuclear power plant in 1994 (Colard, 1995). A similar method has also been used more recently by Tractebel Energy Engineering for evaluating and refurbishing two control rooms in an experimental nuclear power plant. The second methodology is more dedicated to computer screen displays of highly interactive business oriented applications. It also relies on a contextual task analysis in order to generate user interface presentation as automatically as possible in a multi-windowing environment. It has been successfully applied by the University of Namur for several case studies (e.g., remote order, flight reservation, activities data management). 1
Now at Université catholique de Louvain (UCL), http://www.isys.ucl.ac.be/bchi/members/jva
Although both methodologies have been defined independently, they reveal some common properties. For this purpose, their structural similarities, their advantages and disadvantages, the associated work load and required resources (e.g., human work force, time) will be compared. DESIGN OF MIMIC DISPLAYS FOR PROCESS CONTROL The context - the « Doel 3 » nuclear power plant - consists of a dynamic process control situation where operators interact with the process by means of hardware panels and a central desk where the most regular or critical operations are performed. These man-machine interaction surfaces include the controls and indicators required for achieving the tasks. Some computer screens have been added recently to this classical control room setting. The DIMOS informational support system provides operators with synthetic information about the process: e.g., event and alarm lists, mimic displays with graphical representations of important variables and subsystems or devices (e.g., pumps, valves, tanks,...), including their current status, their properties and their inter-relationships. DIMOS is a pure informational system since it does not provide any « soft » controls (Degani & al, 1992) for acting on the process by means of interactions with the screens. Methodology The methodology has been devised to produce a series of mimic images for supporting operators activities related to specific subsets of tasks. In the Doel 3 case study, it has been applied to the tasks associated with the handling and control of the CV circuit, the circuit where volumetric and chemical control of the primary coolant fluid is performed. Tasks include normal or regular tasks, as well as incidental and accidental tasks (e.g., recovery procedures). The classical approach to mimic images design starts with the plans or scale drawings of on-site installations: the plans of the different circuits are simplified, unnecessary information is dropped, and an abstract view of installations is presented to operators. This is the « circuitoriented » approach to mimic images design. A problem with this approach is that it relies on an engineering abstraction of the process, seldom related to the kind of mental models operators needs in real situations. Moreover, the segmentation of the process representation into non taskrelated parts forces operators to navigate or even repetitively alternate between images when information needed for performing a specific task is located on different images. This methodology attempts to address these problems. It relies on a situated and task-oriented approach to mimic images design: tasks of operators are analyzed in the context of their work for devising a series of new images adapted to the activities as they are performed in real operational conditions. The methodology is structured and semi-formalized: it consists of a temporally organized structure of steps, using abstract and precise formalisms whenever possible. Specific ergonomic principles have also been defined to be applied at each step of the methodology: 1. A contextual task analysis where the different tasks operators have to perform are elicited and characterized in terms of three dimensions: type (monitoring, normal and regular, diagnostic, recovery), frequency of execution and complexity. 2. An expression of the task model as a Plan-Based Description (PBD) where a graphical description of the tasks is produced in a plan-based formalism depicting temporal (sequential/parallel) and hierarchical relationships between task elements (fig. 1). 3. An identification of informational needs and their representation in a graphical formalism: using the output of step 2 the information required for achieving each task (mostly process variables) are identified and labeled into five mutually exclusive categories: triggering, pre-
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condition, specification, primary (controlled) and secondary (side-effects) variables. The different variables are then organized for each task in a representation which mirrors the aspect of mimic images (fig. 2). An identification of presentation units and the subsequent allocation of tasks: it consists of determining the number of different images necessary for supporting the whole set of tasks and the allocation of the tasks - and their related information - to the images. Each image is thus designed for supporting a specific set of tasks. Practically, this means finding tasks that can be grouped together and supported by the same image. It mostly involves determining common informational needs between tasks. The graphical representation of informational needs (output of step 3, see fig. 2) proves to be a very valuable tool for achieving this step. An accessibility analysis: it consists of determining how navigation between the different images must be implemented (images are usually organized hierarchically). Vertical links (navigation between the different levels of the hierarchy of images) and horizontal links (navigation within the same level of the hierarchy of images) are thus determined. The selection of informational objects: determination of icons, formats and color codes for representing devices (e.g., pumps), quantities (e.g., digital/analogical values), functional groupings of information, functional links between variables and pointers to other images. The placement of informational objects: information are spatially organized within their respective images.
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Fig. 2. Its associated informational needs.
Examples of ergonomic principles Ergonomic principles have been defined for each step of the methodology. Here are a few examples of principles for some steps or sub-steps of the methodology : • task allocation: allocate tasks that must be performed in parallel to the same image ; allocate tasks of different frequencies to the same image (this will help to maintain familiarity with the information related to tasks that are rarely performed). • selection and placement of informational objects (IO): use the same conventions (e.g., icon families, colors, sizes,...) for representing IO belonging to the same categories ; define isomorphic hierarchies for classifying devices and variables and their corresponding IO ; group functionally related IO within the same portions of the image ; rely on easily detected geometrical features (e.g., lines, matrices,...) for organizing IO that are functionally equivalent or belonging to the same category (e.g., three identical pumps arranged in a row). • accessibility analysis: connect any image supporting a diagnostic task with all the images covering the possible recovery tasks that can be activated as an outcome of the diagnostic task; connect images that support tasks that must be performed in sequence.
Principles have sometimes been shown antagonistic. The designer is therefore responsible for taking these conflicts into account by balancing and prioritizing the principles according to their context. DESIGN OF SCREEN DISPLAYS FOR INTERACTIVE APPLICATIONS The context is limited to highly interactive business oriented applications (e.g., data management, stock exchange, office automation, administration). The global methodology to develop such applications is called TRIDENT (Bodart & al, 1995). It is divided into a methodological framework and software tools that support some steps of the global methodology. Design of screen displays (or Presentation Design) is one of the three fundamental pillars of the TRIDENT methodology (the other two being Conversation Design and Software Architecture Design). Methodology The method consists of performing the following steps: 1. A contextual task analysis where any interactive task is described as a decomposition of subtasks, sub-sub-tasks,…, ending up with actions performed on task objects. Though any task analysis method can be used, Task Knowledge Structure (TKS) method is preferred and generally applied as it can identify tasks, knowledge and structures required for achieving the task (Johnson, 1991). 2. An expression of the task model as an Activity Chaining Graph (ACG) where tasks’ actions and objects respectively become functions and information combined with appropriate relationships; this transformation involves traditional mechanisms such as abstraction, specialization, generalization, composition and decomposition. 3. An identification of presentation units: a presentation unit represents an information input/output world made up of logical windows that are not all necessarily displayed simultaneously in order to carry out a particular sub-task of the main interactive task. 4. An identification of logical windows that are ergonomically related to the ACG physical configuration: for each presentation unit, a set of logical windows is designed. 5. The selection of interaction objects (or widgets like push button, icon, list box, radio button) : for each information to input/display that appears in each logical window, one or several combined interaction objects is selected according to interaction object selection rules; 6. The placement of interaction objects selected in step 5 within the frame of windows identified in step 4; this placement on three semi-orthogonal dimensions: location (where interaction objects should be placed?), size (what are the dimensions of each individual object, e.g., length, height?), and arrangement (what is the logical order of individual objects when they need to be manipulated to carry out a particular task?). Figure 3 depicts the results on this methodology applied for a moderately complex interactive sub-task (the donators’ management for a non-profit organization): this sub-task is presented within one presentation unit divided into three logical windows. The first logical window (fig. 3a) shows personal data with information grouping; official data for the same person are gathered in a second logical window (fig. 3b) that can be obtained by clicking on the second icon in the lower right part of the screen; administrative data (fig. 3b) are located in a third window by clicking on the third icon in the same column. Each window is ended up with four push buttons enabling the user to easily browse the data file (e.g., First/last, Previous/next record). Examples of ergonomic principles The above methodology explicitly involves several ergonomic rules for • Identification of presentation units: a presentation unit is identified for each sub-task of the main interactive task.
Figure 3. Example of a presentation unit consisting of three logical windows (a), (b), (c). • Identification of logical windows: five identification types can be explored: maximal identification (only one window is identified by covering all external input/output information for all functions present in the presentation unit), minimal identification (a window is identified for each external information), input/output identification (a window is identified for all input information of all functions having external information and another window for all output information), functional identification (a window is selected for all information of all functions having external information), free identification (a window is identified for every information set resulting from a partition of the presentation unit information set). • Selection of interaction objects: select an edit box for each information whose domain is unknown ; if domain is known, select radio buttons if the information holds 2 or 3 possible values; surround radio buttons by a group box if the number of possible value is between 4 and 7; select a list box if there are between 8 and 50 values ; prefer a scrollable list box if more than 50 values should be manipulated together. • Placement of interaction objects: apply horizontal or vertical uniformity for input/output interaction objects ; apply proportional or total equilibrium for lines and columns of push buttons for each dialog box ; consecutive group boxes can resized to preserve uniformity ; interaction objects can be arranged according to logical order, frequency order, physical properties.
CONCLUSION Comparing these two methodologies allows us to draw a series of common properties and shared structure. The following features and potential benefits have been identified: • Each methodology attempts to decompose visual display design in a top-down manner : it begins with determining the source from which visual displays will be derived ; it follows with determining how boundaries can specify visual containers (mimic image for the first, presentation unit for the second) ; it ends up with a progressive refinement of these containers to medium objects (interaction surfaces in the first case, logical windows in the second) and individual interaction elements (informational objects and interaction objects respectively). • Each methodology therefore structures a larger problem into smaller pieces with a series of well defined steps ; this similarity could even be improved if the DIMOS system of the first approach was upgraded with the ability to directly control processes through visual screens, which is not currently the case. Indeed, in this situation, individual interaction elements would not be solely restricted to purely informational objects, but could be extended to match a particular form of interaction object; • Identical goals and sub-goals are pursued in equivalent steps of both methods, but they consider different aspects; moreover, both methods starts from a specific form of task analysis to avoid visual design that is purely functional and not operational. Bad design occurs when informational objects are placed together just for the sake of complying with simplified physical circuits (first methodology) or when interaction objects are chosen only because they match all possible functional tasks like insert, delete, search, list, browse (second methodology). • Both methodologies include ergonomic principles and rules that can be exported from one methodology to the other by abstracting their terminology. In other words, underlying ergonomic principles are similar, but they are declined specifically. A common structure is shared by both methodologies: • A preliminary step where a contextual task analysis is performed and task data are gathered and formalized. • A step for determination/segmentation of interaction surfaces (mimic images versus logical windows) • A step for allocating individual interaction elements (informational versus interaction objects) within interaction surfaces; • A step for structuring these individual interaction elements within the frame of interaction surfaces. REFERENCES BODART, F.; HENNEBERT, A.-M.; LEHEUREUX, J.-M.; PROVOT, I.; VANDERDONCKT, J.; ZUCCHINETTI, G. (1995). Key Activities for a Development Methodology of Interactive Applications, Chapter 7 in « Critical Issues in User Interface Systems Engineering », D. Benyon and Ph. Palanque (Eds.) (Springer-Verlag, London). COLARD, M.I. (1995). Méthodologie de conception d’images de conduite. Rapport Final. R & D-ERG/4NT/23/00, Ed. 950321. Confidential. DEGANI, A., PALMER, E.A. and BAUERSFELD, K.G. (1992). « Soft » Controls for Hard Displays : Still a Challenge. Proceedings of the Human Factors Society, 36th Annual Meeting Conference (pp. 52-56). : Human Factors Society. JOHNSON, P. (1991), Human-Computer Interaction - Psychology, Task Analysis and Software Engineering (McGraw-Hill, New York).
