Differentiate functional and non-functional requirements with user research examples.

WHAT THIS TESTS: Whether you can decompose a qualitative user need into two distinct requirement categories that shape different engineering artifacts. Functional requirements define what the system does and appear in system design. Non-functional requirements define how well the system does it and appear in system architecture because they are architecturally significant. The interviewer wants to see that you understand this separation drives implementation planning.

A GOOD ANSWER COVERS: First, a crisp definition: functional requirements specify specific behaviors or functions, while non-functional requirements specify criteria for judging operation. Second, the user research link: both must be derived from the stated need, not invented in a vacuum. Third, concrete examples tied to the prompt. Fourth, the implementation implication: functional requirements are detailed in system design, non-functional requirements in system architecture. Fifth, measurability: non-functional requirements must be testable, not just adjectives.

COMMON WRONG ANSWERS: Conflating the two by saying the feature is fast, which is not a behavior definition. Offering vague non-functional requirements like good performance or easy to use without numbers. Describing only the user interface mockup without separating behavior from quality. Claiming non-functional requirements do not come from user research, when users explicitly state needs like quickly. Ignoring the architectural significance of non-functional requirements.

LIKELY FOLLOW-UPS: How would you validate that the 200 millisecond target is the right threshold. What happens when a non-functional requirement conflicts with a functional one, such as real-time search slowing down the page. How do you trace non-functional requirements through to acceptance criteria. Can a single user story contain both types, and how do you keep them distinct in a backlog.

ONE CONCRETE EXAMPLE: For the need I want to find my past orders quickly, a functional requirement is the system shall provide a search field on the order history page that filters results by date range, product name, and order number. A non-functional requirement is the system shall display filtered results within 200 milliseconds for 95th percentile of requests under a load of 1000 concurrent users. The first drives a UI component and backend endpoint design. The second drives indexing strategy, caching layers, and database architecture.