This paper explores a flaw in traditional design decision making (including optimization) when uncertainty is present. It introduces the concept of the Non-Intuitive Dependent Decision Making (NIDDM) problem, where the assumption that each of the design alternatives is independent can be false due to a common underlying uncertainty. In this situation, the implicit assumption that design alternatives are independent can lead to a sub-optimal selection. This paper provides a simplified example of the NIDDM problem, and uses this to define the conditions where the NIDDM problem arises. An aerospace design toolset is then used to explore the NIDDM problem in realistic conditions, and a discussion is presented about when traditional robust design processes are appropriate or an alternative design methodology is needed.

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