Identify what customers want in your solution.
Functional requirements, and the degree to which they are satisfied, form the basis of customer satisfaction with your products and services. For example, people who drink diet soda expect it to have a certain taste and caloric content. If the soda fails in either regard, customers may move to a competing solution that better meets their expectations.
Just as outcome expectations (see Technique 3) provide solution-neutral hiring criteria to get a job done (see Technique 1), functional requirements provide solution-specific requirements against which designs are created. Therefore, functional requirements are a direct translation of outcome expectations with a specific solution in mind.
Designing the details of the solution begins with understanding and articulating functional requirements from the customer's point of view. The requirements provide limits on two types of functions addressed in the design—useful functions and harmful functions. As an example, the degree of brightness displayed by a candle is a useful function, whereas the amount of soot produced is a harmful function. So the candle design functional requirements will provide the targets and limits for amount of brightness and soot production.
To successfully establish the functional requirements, you'll need at least a modest understanding of how to gather voice-of-the-customer data via surveys, focus groups, and interviews.