Defining Changeability

Designing and maintaining systems in a dynamic contemporary environment requires a rethinking of how systems provide value to stakeholders over time. Developing either changeable or classically robust systems are approaches to promoting value sustainment. But, ambiguity in definitions across system domains has resulted in an inability to specify, design, and verify to ilities that promote value sustainment. In order to develop domain-neutral constructs for improved system design, the definitions of flexibility, adaptability, scalability, modifiability, and robustness are shown to relate to the core concept of “changeability,” described by three aspects: change agents, change effects, and change mechanisms. In terms of system form or function parameter changes, flexibility and adaptability reflect the location of the change agent—system boundary external or internal respectively. Scalability, modifiability, and robustness relate to change effects, which are quantified differences in system parameters before and after a change has occurred. The extent of changeability is determined using a tradespace network formulation, counting the number of possible and decision maker acceptable change mechanisms available to a system, quantified as the filtered outdegree. Designing changeable systems allows for the possibility of maintaining value delivery over a system lifecycle, in spite of changes in contexts, thereby achieving value robustness.