## 4 Results and Discussions
### The Decision-Making Trial and Evaluation Laboratory (DEMATEL) Analysis  
This study developed the DANP expert questionnaire based on the results of the fuzzy Delphi analysis to explore the cause-and-effect relationship and correlation between the product design indicators of wearable devices. First, based on the total impact relationship matrix of criteria and indicators, we calculated the column sum and row sum of the total impact relationship matrix T. This study used d + r (correlation degree) as the abscissa and d - r (cause degree) as the ordinate to form a cause-and-effect diagram, in which the criteria and indicators of product design for wearable devices were plotted. In addition, the total impact relationship matrix was simplified based on the criteria and indicators, and then the correlation between the criteria or indicators was depicted in the cause-and-effect diagram to explore whether the impact relationship between the criteria or indicators was one-way (black index line) or two-way (red index line). In summary, the criterion cause-effect diagram and the index cause-effect diagram (Figure 3) of the product design indicators for wearable devices developed by this study are explained as follows:  
1. Criteria  
In terms of correlation degree, the criteria were sorted by the correlation degree (d + r) in order, including \"Personalized Design\", \"Function Enhancement\", \"Device Extension\", \"Future Outlook\", \"Quality Appearance\", and \"Bio-Friendliness\". Among them, the correlation degrees (d + r) of the \"Personalized Design\", \"Function Enhancement\", \"Device Extension\", and \"Future Outlook\" criteria were all greater than the average. Therefore, it is learned that they belong to the critical factors of greater importance among the developed product design criteria for wearable devices. In terms of cause degree, the cause degrees (d - r) of the criteria \"Bio-Friendliness\", \"Personalized Design\" and \"Device Extension\" were less than 0, such that they belong to the factors that are more easily affected, that is, the \"effect\" in the cause-and-effect relationship. Regarding the criterion \"Quality Appearance\", its correlation degree was not much different from the average, and the cause degree was greater than zero, so it belongs to the cause criterion, that is, the \"cause\" in the cause-and-effect relationship. However, the correlation degrees of the criteria \"Function Enhancement\" and \"Future Outlook\" were greater than the average, and their cause degrees were greater than zero as well. It is clear that these three criteria belong to core criteria which have more active impact in the construction of product design indicators for wearable devices. Through the enhancement of their functions or quality, not only can they help boost the effectiveness of other criteria or indicators, but they can also help achieve the purpose of increasing consumers' willingness to use the product. Therefore, it is recommended that businesses which intend to develop wearable devices should pay more attention to the criteria of product design, such as \"Function Enhancement\", \"Future Outlook\", and \"Quality Appearance\", or their corresponding indicators. 1. Indicators  
In terms of correlation degree, the indicators based on the correlation degree (d + r) were sorted in order as \"Intelligence\", \"Customization\", \"Convenience\", \"Interactivity\", \"Foresight\", \"Reliability\", \"Connectivity\", \"Accuracy\", \"Integration\", \"Compatibility\", \"Security\", \"Energy Consumption\", \"Privacy\", \"Sustainability\", \"Comfort\", and \"Appearance\". Among them, \"Intelligence\", \"Customization\", \"Convenience\", \"Interactivity\", \"Foresight\", \"Reliability\", \"Connectivity\", \"Accuracy\", and \"Integration\" were the indicators whose degrees of correlation (d + r) were greater than the average, so they belong to critical factors of greater importance.