What is the difference between a functional and a non-functional requirement?
Assesses fundamental understanding of Business Analysis conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
Functional requirements describe what the system does: behaviours, features and business rules. Examples include calculating a discount, generating an invoice or allowing a user to reset a password.
Non functional requirements describe how well it does it: quality attributes and constraints. Examples include performance, security, availability, scalability, usability, compliance and accessibility.
Both matter, but non functional requirements are often skipped and then cause expensive problems later. A feature that works but takes ten seconds to load, fails under load, or is inaccessible is not acceptable.
They should be specific and measurable, or they cannot be tested. Instead of "the system should be fast", write "the search results page must load in under two seconds for ninety-five percent of requests at peak load".
I capture them early, include them in acceptance criteria and trace them to tests so they are verified, not assumed.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.