What's in a prerequisite [learning environments]

It is important that all pedagogical design decisions in educational learning environments are made explicit to the educator. However, implicit assumptions affecting or even contradicting the "official" pedagogical method may unbeknownst slip into the design. We discuss a concrete problem in the area of adaptive hypermedia systems related to the notion of a prerequisite. Prerequisites are frequently used to describe the many different educationally effective paths on which learners may traverse a hypermedia system. However, a closer look reveals that several different concepts are called prerequisite and confusing them may lead to flawed designs. We make recommendations as how to avoid misusing the different notions of prerequisites in adaptive hypermedia systems.