*2.3. Fuzzy Theory*

The fuzzy theory bridges the gap between the precise classical mathematics and the real world that is full of vagueness. When encountering a problem with a high degree of vagueness and uncertainty, the fuzzy set of membership function can be used to quantize the perception of designers and consumers, as it is known that, di fferentiation can be realized only by comparison. During production, scientific research and our daily life, people always compare things and evaluate their quality in order to determine the optimal way of handling them. These activities include the evaluation of a member's quality, the quality of goods and the evaluation of how reasonable certain engineering design parameters are. Since a thing/object has various attributes and is affected by various factors, carrying out comprehensive consideration and fuzzy factors on several relevant factors during the process of evaluating things/objects is required. This process is called the fuzzy comprehensive evaluation.

#### *2.4. Product Assembly Design Case*

In order to find the suitable and applicable design cases and to describe the development of research methodology in a clearer way, the positioning of the product attributes in this study is on those products that are at the mature stage of product life cycle. The design consideration highlights the manufacturing and assembly orientation. Therefore, stereo systems that are available on the market were selected for the case study. Similar to other home appliances, a stereo system is assembled by screws or rivets and it is expected that the result can serve as a good reference for other relevant studies.

#### **3. Research Process and Methods**

In this study, the disassembly, ranking and evaluation approaches for assembly designs were applied to the target product. To start with the process, component drawings were processed to form the component matrices. After the ranking of assembly evaluation, the simulated scenarios of users can be developed by the usability engineering and the way how the participants were using the assembly was recorded. The fuzzy theory was implemented to the evaluation of product assembly and the degree of user preference was compared an analyzed. The purpose was to determine the correlation between the assembly consideration from the engineering orientation and the usability consideration from the user orientation. The flowchart and framework of this study is shown in Figure 2.

**Figure 2.** Flowchart and framework of this study.
