Features for reusability Selection . The need for retrievability entails that authors must be able to select, amongst a set of possibly reusable components, the one which best suits their needs. With this respect, the flexibility of an approach based on database techniques can provide the author with a valuable aid in indexing and retrieving existing artifacts. In particular, for what concerns the identification of the content, a very critical aspect lies in the usage of the same terminology and keywords, which can be induced by supporting both the indexing and the retrieval with word lists and thesauri. As far as multimediality is concerned, a possible approach to the indexing of a multimedia document consists in producing a surrogate of the primary document, i.e. a textual, abstract-like description of it, that can be analysed with classical indexing methods.
Abstraction supports both retrievability and adaptability.
We can adapt this definition into the amount of intellectual effort required to an author to understand the features of a given reusable artifact . The availability of surrogates which support the author in the selection of the material to reuse fits the needs for abstraction too, in particular for complex courseware artifacts. For example, a good surrogate of an audio-visual sequence or a tutorial can be seen as a high level view on the learning material, which provides the basis for a relevance judgment saving the time of the complete fruition of the material. When considering courseware artifacts other than presentational material, such as knowledge representation graphs, student models or other products of the courseware development process, the intrinsic structure of the artifact is usually so complex that abstraction is definitely necessary to make reuse effective. Once a component has been identified and selected, it must usually be adapted to fit in a new context.