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INTERVIEW QUESTION:

What distinguishes a functional requirement from a solution or design constraint?

Posted by Adrian M.

// 285 Views // 0 Additional Answers & Comments

Categories: Requirements Analysis (BABOK KA), Requirements Management and Communication (BABOK KA)

ANSWER

Functional requirements describe what the system must do, while solution or design constraints set the boundaries on how the system may be built.

Definitions

Functional requirement (FR): An externally visible behavior that satisfies a business need. Example: “The system shall calculate the total order price, applying state-specific sales tax and any active promotions.
Solution / design constraint (SC): A rule or limitation that restricts technology, architecture, process, or environment. Example: “The calculation must reuse the existing SAP tax engine and complete in ≤ 2 seconds.”

Key Differences

Aspect	Functional Requirement	Solution / Design Constraint
Purpose	Deliver a capability or service	Limit the implementation space
Focus	Outcomes, interactions, user value	Tools, standards, timing, budget, compliance
Validation	User acceptance tests, use-case walkthroughs	Design reviews, architectural conformance, audits
Flexibility	Multiple ways to meet the need	Narrows or eliminates options

Why It Matters

Clarity: Separating FRs from constraints prevents “design by requirement,” keeping the what independent of the how.
Innovation: Teams can explore creative solutions as long as they respect immovable guardrails.
Traceability & Testing: FRs map to user acceptance tests; constraints map to design or non-functional verification, reducing gaps at audit or release time.

Illustrative Scenario

A retailer wants faster checkout.

FR: “The POS system shall apply loyalty discounts and display the final total to the cashier.”
SCs:
1. “The solution must run on existing Android POS tablets.”
2. “Data transmission must be encrypted to meet PCI-DSS requirements.”
3. “Discount calculation response time shall not exceed 1.5 seconds.”

The FR articulates the desired capability, leaving room for various algorithms or UI designs; the SCs fence those choices with technology, regulatory, and performance limits.

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Do your homework prior to the business analysis interview!

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