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The applicability of multimedia databases in education may be extended if they can serve multiple target groups, leading to affordable costs per unit for the user. In this contribution, an approach is described to build generic multimedia databases to serve that purpose. This approach is elaborated within the ODB Project ('Instructional Design of an Optical DataBase'); the term optical refers to the use of optical storage media to hold the audiovisual components. The project aims at developing a database in which a hypermedia encyclopedia is combined with instructional multimedia applications for different target groups at different educational levels. The architecture of the Optical Database will allow for switching between application types while working (for instance from tutorial instruction via the encyclopedia to a simulation and back). For instruction, the content of the database is thereby organized around so-called standard instruction routes: one route per target group. In the project, the teacher is regarded as the manager of instruction. From that perspective, the database is primarily organized as a teaching facility. Central to the research is the condition that the architecture of the Optical Database has to enable teachers to select and tailor instruction routes to their needs in a way that is perceived as logical and easy to use. Keywurds: Adaptability; Instructional database; Multimedia; Object-oriented approach; Teacher-support systems. Context of the current ODB project The seemingly unlimited possibilities that multimedia systems offer for presenting information in any desired format, and the interactive capabilities that stem from the built-in computer power offer, in principle, a vast range of educational applications. Traditional courseware options (e.g. Alessi & Trollip, 1985; Jonassen, 1988), types of interactive video (e.g. Bayard-White & Hoffos, 1988), instructional uses of computer simulations (e.g. De Jong, 1991; Van Schaick-Zillisen, 19901, electronic books and hypertext applications (e.g. Correspondence: P h W. Verha en, Faculty of Educational Science and Technolo y, University of Twente, P.O. Box 217,7500 Ab Enxhede, Netherlands. Email: verhagen@e%e.utwente.nl 81 82 P.W. Verhagen and R. Bestebreurtje Barker & Manji, 1991; Benest, 1991; Megarry, 1988, 1991; Kommers, 1991; Kuhlen, 1991) may be combined into one system so that education can benefit from the traditional options and from the multimedia opportunities. The current ODB Project focuses on multimedia databases for normal education. A multimedia platform is in fact a single medium (the computer) that is purported to be multiple. The term 'multimessage system' would perhaps have been more applicable, as Copeland (1991) suggests. Here, however, the now generally accepted term 'multimedia' will be used in the sense just described. From the perspective of developing an architecture of an instructional multimedia database, the following problems need to be solved before the attractive features of the new technologies can be exploited (Bestebreurtje & Verhagen, 1990). TG applicability of large multimedia databases depends on the retrievability of the required information to cover specific needs. The form and level for presenting that information should then match the capabilities of the user and the desired pattern of interaction (inform me, teach me, challenge me, etc. see Megarry, 1988). Design methods to accomplish this in a systematic way are yet to be developed, in particular if tuning to a variety of target audiences is required. There is reason to believe that, at least with respect to video, traditional audio-visual design principles will have to be altered before they can be applied (Locatis ef al., 1990). The production costs of multimedia applications are high, in particular if video components are part of it. Putting these media into practice for normal education will be dependent on a large number of copies in order to make the price per title affordable to the schools. However, in education target groups are usually relatively small. At the secondary level for instance, the number of schools of one type is, in many countries, too limited to allow for volume production. True (and economical) multiple use will only occur to the extent that different groups can use the same data, which has its main effect for audio and video fragments. The former problem is reinforced by technical constraints: although optical media such as videodisc and CD-ROM have a relatively large storage capacity, the total size of the storage capacity is limited. This sets bounds to extending a multimedia data collection for multiple use. Methodological knowledge is needed to specify the conditions for completeness of an application in such a way, that multiple use can be maximized within the limited space of the available storage medium. 'Completeness' is thereby to be understood as the capacity of the system to treat each available subject in such a way that the specified instructional objectives for each target group can be reached without a need for information other than that contained in the system. It has to be emphasized that this does not limit the use of the system to learning from the screen. System-generated study texts and worksheets, which Architecture of an instructional multimedia database 83 will be made available to the teachers, may very well be part of instructional situations. Education is primarily a social process between teachers and learners. In normal schools, the teacher has a central role in this process, being the manager of education. For multimedia applications to become acceptable they have to fit into this situation. This means that first of all teachers should recognize the applications as valuable tools for their teaching. Putting the teachers in a central position means making them partners in developing didactics and organizational patterns for the application of multimedia databases in education. In this process attention is also directed to gain experience in using a multimedia database in the school setting. This kind of experience is rare, as witnessed by the limited number of case studies that are regularly reported in the literature (eg. Helsel, 1990). In this paper emphasis will be placed on the requirements and the development of the architecture of the Optical DataBase and more specifically on its functional specification. This leads to the model that is presented in Fig. 1 where the main characteristics of the object-oriented database architecture that is developed for the project are summarized. The intended end-product is an experimental database for multiple use, that is organized for educational purposes for a limited domain of knowledge and for a small but sufficient number of target groups in order to make the study feasible. Overview of the end-product: an instructional database The content of the Optical DataBase is based on actual curricula on cheesemaking in Dutch agricultural vocational schools with food science as a field of study. Its aim is to provide these schools with a knowledge base on cheesemaking with a degree of completeness for instructional purposes at all relevant educational levels. Types ofapplications In this educational context 'multiple applications' means uses for (1) classroom presentations; (2) individual or small group instruction (tutorial, inquiry, problem solving); (3) consulting of encyclopedic information; and (4) managing the former three. The central role of the teacher should be supported here by his or her ability to modify the database, monitor progress of learners with help of the system, and so on. The database will be organized to offer these possibilities in a coherent way, such that switching between application modes will be possible as often as desired (and when allowed, teachers will have rights that are withheld from learners). The access to the knowledge domain is done on the basis of a landscape metaphor. The user travels through the knowledge domain along instructional routes by which the knowledge landscape is opened up, thus 84 P.W. Verhagen and R. Bestebreurtje amving at one topic after the other. Switching between application modes means turning to a different route along which the relevant subject is treated from a different perspective. Transfer over larger distances will be made possible by building-in navigation mechanisms as used in hypertext and hypermedia applications.