To achieve a universal design that satisfies diverse user requirements associated with aging and internationalization, designers must make a decision based on diverse user requirements. Designers have generally incorporated average human physical characteristics in their designs. Thus, user limitations are critically important. Traditional design methods often regard engineering and product design as iterative processes based on point values. However, when user information is represented as a point value, the resulting product satisfies only that specific user group and does not necessarily satisfy diverse user groups. This study proposes a universal design method that obtains diversely ranged design solutions for user requirements. The proposed method defines diverse user requirements, design variables, and user characteristics as sets, which range in value. To represent user information accurately, users are classified into numerous groups using classification techniques. Design variables are divided into two types: control and noise. Control factors are designer-controllable variables that are based on design specifications. Noise factors are designer-uncontrollable variables representing user characteristics. To derive a ranged design solution set, designers clarify the relationship between performance and design variables. Ranged solutions satisfying required performance are derived for each group using all relational expressions and ranged variable values. The combinations of divided design variables that cannot satisfy the required performance are eliminated from the design proposal, and the narrowed range of design variables become ranged solutions. The ranged solutions are derived for each group, and the common range of design variables is the ranged solution for all users. This paper chooses the design problem of the strap height of a train as a case study of the proposed universal design method. In this case study, we consider diverse user requirements based on the variability of physical characteristics. This paper discusses the suitability of our proposed approach for obtaining ranged solutions that reflect the physical characteristics of diverse users.

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