Designing for Variability: From Stable Assumptions to Adaptive Digital Systems

Digital systems are almost universally designed under the assumption of environmental and user ability stability, often being built for the "ideal" user. Yet, the reality of everyday interaction often using multiple devices (desktop and mobile) is a chaotic mix of needs and abilities, disruption, multitasking, and shifting constraints, emotions and situations. System failure during these moments of friction reveals fundamental misalignment between design theory and the lived experience of the user. In this paper, we argue for a paradigm shift from designing for the assumption of capacity and stability, to designing for the reality of all users, their diversity and variability, and the dynamic real-life contexts. Instead of treating accessibility and inclusion as a secondary compliance measure, we position shifting variability as the core condition of design. We argue that by designing for a "variable" user, we inherently create an inclusive system. To operationalize this perspective, we integrate a set of variability-oriented design principles into the Unique Centered Design (U*CD) framework, a practical toolkit that helps design teams identify potential functional mismatches and anticipate exclusionary design decisions early in the development process. The usability of the proposed framework is assessed in a game design case study, which is conceived as a multi-stakeholder semi-automated co-creation process. The use case demonstrates how the toolkit stimulates discussion and supports comprehension around user variability and encourages design of games that reflect social and cultural diversity.