Merrill and Tennyson have designed a model for teaching concepts which is based upon a set of propositions derived from prior research. Their model includes a definition expressed in term of critical attributes . an expository presentation of matched examples and non examples, were matched means similarity of variable attributes, where matched means similarity of variable attributes, and an inquisitor practice during which students are required to different between unfamiliar examples and non-examples and to justify their choices in terms out of the definition of the concept. The Merrill and Tennyson model also specifies that examples/ non examples in the expository presentation should be sequenced from easy to difficult and should be divergent .
This model is admirable as a serious attempt to build theory which can guide instructional practice.