A feature of good modularity is the ease of changing a module within a product. Existing modularity methods use subjective or qualitative attributes to evaluate architectures. We develop a method to relatively compare proposed product architectures according to design complexity. Our metric represents the difficulty that different module boundary interactions, represented by flows in and out of a function, would have in terms of redesign effort. We decomposed medical injector head systems and conducted interviews in two companies to find out a relative redesign effort for various interaction types, e.g. electrical and mechanical connection, signal flows, etc. We found that to change a flow by 1%, 1–4% more design effort is required, depending on the interaction type. We also found that decreasing a flow value causes, in general, less rework than increasing a flow. Our metric proved to be a valuable tool in estimating the redesign difficulty of an architecture.

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