Terry Anderson in the article Getting the Mix Right Again: An updated and theoretical rationale for interaction wants "to provide a theoretical rationale and guide for instructional designers and teachers interested in developing distance education systems that are both effective and efficient in meeting diverse student learning needs" (1). The author starts out his debate from the premise that interaction has an essential importance in the education and learning process, including the forms delivered at distance. After briefly reviewing some functions of the interaction given by previous critique, Anderson, based on personal observations and the literature debate, defines an equivalency theorem as follows: "Deep and meaningful formal learning is supported as long as one of the three forms of interaction (student-teacher; student-student; student-content) is at a high level. The other two may be offered at minimal levels, or even eliminated, without degrading the educational experience [...]" (p. 4). After that, Anderson discusses individually the characteristics of three most common types of interaction (student-student, student-teacher, student-content) to which he adds other three types of interaction: teacher-teacher, teacher-content, content-content. The author then applies this equivalency theorem to different environments (classroom delivery, "traditional distance education delivered via mail or electronic correspondence", audio and video conferencing, web-based courses), assessing the level and the kind of interaction that takes place in each case. Anderson concludes by formulating an "interaction-based model of e-learning" which involves shifting the equivalency theorem towards a substitution one: "Getting the mix right involves a series of tradeoffs, and knowing how one type of interaction can effectively substitute for another, provides an essential decision making skill in the distance educators' knowledge base" (p. 11).
Personally, I agree with Anderson's equivalency theorem, although I would ideally tend to see it not as one interaction successfully replacing the other(s), but more in terms of equal amount of time dedicated to each of the three main interactions: student-teacher, student-student and student-content, in order to achieve a high level of education and learning. But I also understand that an equal amount of time for each interaction is not always necessary (it depends on the goals, the type of courses, the skills that are to be developed etc) or possible, especially in a CALL environment. Also, in his equivalency theorem, Anderson defines - at least in my reading - a high or low level of interactivity in terms of quantity. I think it would be interesting to see how this picture would change in case we shift the importance on quality. What happens if a student has mainly interaction with content, therefore student-content interaction is quantitatively high, but not also qualitatively, for whatever reason?