Criteria: Functional Model

In my previous post, I said that “Functional design is not just adding up the list of functions …, but rather organizing them into a lean, mean performing machine.“.  Functional design is a fairly abstract process, so it’s quite helpful to have a set of “concrete” criteria that can be used to evaluate competing designs:

Here are the criteria I use:

These will help you identify the full range of functions that should be included to meet the product’s mission, while keeping the model “lean” (simple, small number of functions and interactions) and “mean” (yields great end-to-end performance in high priority use cases).

Treating functional design as a formal decision can help avoid “tunnel vision” that often occurs if you see this important thinking task as just another modeling effort.  Functional decomposition is a decision; not all functional models are created equal.