Editor: Nicolas Balacheff, LIG, Grenoble, France
The term overlay is extensively used in learner modeling research, especially for tutoring systems where Wenger acknowledges it as a "standard paradigm" (p.137). It has been coined by Carr and Goldstein in 1977 and defined in the following way:
- "The kernel idea is to design a modular Expert program for the task, and to explain differences between the behavior of the Expert and the subject in terms of the lack, on the player's part, of some of the Expert skills. Thus, a model of the player is a set of hypotheses, each of which records the system's confidence that the player possesses a given skill. Such models are called overlays to reflect the fact that the model of the individual is basically a perturbation on the Expert's structure." (p.4)
While "overlay" would mean that one model is in some way covering the other, the definition from Carr and Goldstein restricts this relation in a way which causes certain criticism of the choice of this term:
- "An "overlay model" (Carr and Goldstein, 1977) is one in which the only possible diagnosed discrepancies are missing pieces of knowledge: RS*k ⊃ RSRL*k. The use of the term "overlay" is not ideal because of its ambiguity: "partial model" would definitely be better, but "overlay" has already a long history in ITS." (Dillenbourg and Self 1992)
Learner modeling, Intelligent Tutoring Systems.
French: expertise partielle. The term is often directly imported (overlay)
This term has been coined as a response to the needs of a TEL research project.
 Carr B., Goldstein I. P. (1977) Overlay: a theory of modelling for computer aided instruction. AI Memo 406. Cambridge USA: Massachusetts Institute of Technology
 Dillenbourg P., Self J. (1992) A framework for learner modelling. Interactive Learning Environments, 2 (2) 111-137
 Wenger E. (1987) Artificial Intelligence and Tutoring Systems. Los Altos: Morgan Kaufmann Pub.