*2.2. Usability Engineering*

In recent years, usability engineering has become one of the most important measurement indices of various product and interface designs. Norman [16], a cognitive psychologist, proposed the user-oriented thinking in order to see the whole picture of "human/machine/environment" and "human/product/scenario." He proposed an approach that allowed users to accept a product in a natural way by following his/her own mental characteristics without forcing the users to build a new mental model as shown in Figure 1.

**Figure 1.** Mental model of a stereo system (adapted from Norman [16]).
