Continuity as a usability property

Continuity as a Usability Property Daniela Trevisan1,2, Jean Vanderdonckt2, Benoît Macq1 Université catholique de Louvain Communications and Remote Sensing Laboratory Place du Levant, 2 – B-1348 Louvain-la-Neuve (Belgium) {trevisan,macq}@tele.ucl.ac.be 2 Unit of Information Systems, ISYS/BCHI Place des Doyens, 1 – B-1348 Louvain-la-Neuve (Belgium) {trevisan,vanderdonckt}@isys.ucl.ac.be 1

Abstract In this paper, we describe continuity as an important property to reach usability in many emerging systems such as multimodal and virtual reality systems. The approach is based on synchronization and integration characteristics existent between entities involved in the domain.

1

Introduction

In modern interaction techniques such as gesture recognition, animation and haptic feedback, the user is in constant interaction with the computing system. Thus the interaction is no longer based only on the discrete events but is based on a continuous process of information exchange. This is particularly the case for virtual environments where interaction remains continuous over time. As continuity of interaction may influence usability, it is important to consider it as an additional usability property, beyond existing usability guidelines that are applicable to virtual environments (Kaur, Sutcliffe, & Maiden, 1999) (Kaur, Maiden, & Sutcliffe, 1999). In (Nigay, Dubois, & Troccaz, 2001), continuity is applied at the perceptual and cognitive levels. Perceptual continuity is verified if the user directly and smoothly perceives the different representations of a given entity. Cognitive continuity is verified if the cognitive processes that are involved in the interpretation of the different perceived representations are similar. Here we define the continuity as a capability of the system to promote a smooth interaction scheme with the user during task accomplishment considering perceptual, cognitive and functional aspects. The functional aspects correspond to those discontinuities that can occur between different functional workspaces, forcing the user to change and/or learn new modes of operation.

2

Continuity and usability properties

Gram & Cockton (1996) define a set of user-centred properties of interactive systems, which promote high usability quality from the user’s perspective. The usability property establishes how well the users can interact with the system and meet their goal. Three external properties may to improve system usability: goal and task completeness, interaction flexibility, and interaction robustness. In this approach we are interested in addressing the robustness once that it refers to the user can avoid doing things you wish he/she had not done. Thus, interaction robustness covers all those properties that minimise the risk of task failure as observability, insistence, honesty,

predictability, and access control, pace tolerance and deviation tolerance. To address the continuous interaction according the definition given here we should consider the observability and honesty properties. The observability property evaluates if the system makes all relevant information potentially available to the user. Of course, not all information should be displayed all the time. In this case, browsability suggests that information which is not first-class information required to carry out the task may be accessible on-demand. Therefore, information that is not observable may become browsable. It corresponds to cognitive aspects. The honesty property measures if the dialog structure ensures that users correctly interpret perceived information. It corresponds to perceptual aspects. Thus, we should introduce the functional property to provide a complete analysis of continuous interaction contributing to the principle of interaction robustness. The functional property addresses the discontinuities between different functional workspaces. The next subsection proposes an analysis based on synchronisation, integration. These are characteristics that result from interaction and relationships between all entities involved in the domain and contribute to design system in terms of continuity properties (See Figure 1). The honesty and observability properties involve respectively cognitive and perceptual aspects of continuity (see Table 1).

3

Temporal synchronisation of entities

Synchronisation is an event controlled by system that should be analysed between media, devices and tasks. Basically there are two types of temporal synchronisation: sequential (before relation) and simultaneous that can be equal, meets, overlaps, during, starts, or finishes relations according to description in (Allen, 1993). Regarding media synchronisation, it is possible to find all these kinds of temporal relationships and we can still consider the start- and end-points of events and distinguish the end of the event in natural (i.e., when the media object finishes its presentation) and forced (i.e., when an event explicitly stops the presentation of a media object). Devices synchronisation describes a way that devices will be available to the user interacts with them at a specific time. It raises the question of how the user interaction is with multiple devices. For example, if the system permits to select one object using a data glove and another with speech recognition at the same time, then there is simultaneous synchronisation. Tasks synchronisation can be simultaneous or sequential and performed by one user, by various users, by the system, by a third party, or by any combination.

4

Integration

We have considered three aspects about, which are: physical integration, spatial integration and insertion context of devices. The physical integration is controlled by the system and it describes how the user will receive feedback and how the media are distributed into output devices. It means that each media could be displayed in different displays or integrated within the same display. For example, overlapping real and virtual images in a head mounted display or showing sequences of images in a multiscreen device. Spatial integration concerns the spatial ordering and topological features of the participating visual objects. The spatial composition of objects can be performed by designer or by users or by the system for example in augmented reality systems it come from registration procedures by mixing correct way both information, real and digital. There is a spatial integration between media entities only when they are integrated into the same device. Insertion context of device can be peripheral or central according to the user’s task. If the device is inserted in the central context of the user’s task, she does not need to change her attention focus to perform

the task. Otherwise if the user is changing the attention focus all time, then in this case the device is inserted in context peripheral of use.

5

Identifying Continuity

Norman’s interaction model (Norman & Draper, 1986) show how the seven stage model can be used to ask design questions about the system regarding the human side of the interaction between human and system. In this model is possible to identify two main levels in the execution cycle of a task: execution and evaluation flows. The execution level consists of how the user will accomplish the task involving the temporal interaction synchronisation, the insertion context of input devices in the environment and the operation mode corresponding to the functional aspects. The evaluation level consists of three phases: user’s perception, interpretation and evaluation. The perception corresponds to how the user perceives the system state involving the temporal interaction synchronisation; spatial and physical media-device integration and insertion context of output devices in the environment. The interpretation level consists of how much cognitive effort the user needs to understand the system state. It depends of what communication language or media type will be used by the system to provide the feedback to the user. The last phase corresponds to the evaluation of the system state by the user with respect to the goals.

Synchronization performs

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Spatial Integration

Physical integration

Insertion context

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Figure 1: Continuity analysis based on synchronisation and integration characteristics Norman’s Theory Interpretatio n and Evaluation level

Continuity properties Perception

External properties [2]

Characteristics related

Observability

Cognitive

Honesty

Execution level

Functional

Temporal synchronisation Integration Insertion context of output devices Language and media used to represent the information Temporal synchronisation Insertion context of input devices

Table 1: Characteristics related to continuous interaction properties according to the Norman’s theory.

Figure 3: Example of potential discontinuity. Figure 3 shows a potential source of discontinuity in Image-Guided Surgery (IGS): while a surgeon is operating on patient laying down on a table (the primary task and thus, the main focus of attention), additional information is displayed on TV screens and monitors. Those devices are not necessarily located closely to the patient’s location, thus forcing the surgeon to switch attention from the patient to the various devices and then to come back on the main focus of attention. The more far the devices are from the main focus of attention, the more important the discontinuity may be induced.

6

Discussion

Regarding the model in Figure 1 there is a dependence between task and devices synchronisation that should be respected to improve continuity in the system. Insertion context of devices in the environment according to the user’s task focus is a keep point to provide to the user a continuous functional interaction. The interaction model of Norman provides interaction analyses for systems in which the system state changes as a result of human actions. Therefore is interesting to consider another model such that suggest by (Massink & Faconti, 2002) supporting the analyses interaction from system side too

Acknowledgments We gratefully acknowledge the support from the Special Research Funds of “Université catholique de Louvain” under contract QANT01C6. The work described here is part of the VISME project (VIsual Scene composition with multi-resolution and modulation for a Multi-sources Environment dedicated to neuro-navigation). The web site of this project is accessible at http://www.isys.ucl.ac. be/bchi/research/visme.htm

References Allen, J.F. (1993). Maintaining knowledge about temporal intervals. Communications of the ACM, 26(11), 832–843.

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