The exercise has been designed according to concept learning principles (Engelmann & Carnine, 1982; Grant, 1986; Markle & Tiemann, 1974; Tennyson & Cocchiarella, 1986). For example, in the introductory section the use of matched example/nonexample pairs has been supported by research showing that it is desirable to present the learner with such pairs in which a single critical feature of the concept being taught is missing in the nonexample (Carnine, 1980). Information about other important concept-teaching principles may be found in the above references.