ISO 9241-11 ユーザビリティの定義

The ISO 9241-11 standard, part of the broader ISO 9241 series on “Ergonomics of human-system interaction,” provided a crucial, formal definition that helped mature the field of usability engineering. Prior to its establishment, “usability” was often used loosely. This standard operationalized the concept into three measurable dimensions. ‘Effectiveness’ refers to the accuracy and completeness with which users achieve specified goals. For example, can a user successfully complete a purchase on an e-commerce site? ‘Efficiency’ relates to the resources expended in relation to the accuracy and completeness with which users achieve goals. This is often measured in terms of time taken or clicks required. ‘Satisfaction’ is the user’s subjective response—their freedom from discomfort and positive attitudes towards the use of the product. It is typically measured using post-test questionnaires. The definition also emphasizes the “specified context of use,” acknowledging that usability is not an absolute, intrinsic quality of a product but is dependent on the specific users, their goals, and the environment (physical, social, technical) in which they operate. This contextual element is critical, as a product highly usable for an expert in a controlled office environment might be unusable for a novice in a distracting, mobile setting. The standard’s framework guides practitioners in setting clear, testable usability goals and evaluating designs against them.