Question 4 of 25: Quantifiable Non-Functional Requirements
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
98-Comp-B11 Advanced Software Design — National Exams, May 2014. 3 hours, open book, no calculator permitted. The paper is organized into five parts, and candidates were instructed to answer any three (3) questions in Part I, any four (4) in Part II, any three (3) in Part III, any one (1) in Part IV, and any one (1) in Part V — only the first questions answered, in each part, as they appear in the answer book are marked. All questions carry equal weight, so the 12 questions actually marked (3+4+3+1+1 of 25) each count for 100/12 ≈ 8.3% of the paper. All 25 questions are answered below for completeness.
Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, agile methods, design principles; Pressman, Software Engineering: A Practitioner's Approach (9th ed.) — supplementary process and quality coverage; Gamma, Helm, Johnson & Vlissides (GoF), Design Patterns: Elements of Reusable Object-Oriented Software — structural/behavioural pattern catalogue (Proxy, Bridge, Strategy, Observer, Template Method, Composite, etc.); Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, inheritance and language-level object semantics (also underpins the Java/C++ discussion in Part V). Bertrand Meyer's Object-Oriented Software Construction is cited by name where the paper's own vocabulary (design by contract, open–closed principle) originates there; Barbara Liskov's 1987 substitutability paper is likewise cited by name for Question 11.
A non-functional requirement is "quantifiable" when it is stated as a specific, measurable target with a defined metric and threshold, rather than as a vague quality adjective. "The system shall be fast," "the system shall be reliable," and "the system shall be user-friendly" cannot be objectively tested: different stakeholders will disagree on whether the delivered system satisfies them, there is no unambiguous pass/fail criterion at acceptance testing, and architects have no concrete target to design against.
Quantification replaces the adjective with a testable criterion:
"Fast" → "The search function shall return results within 2 seconds for at least 95% of queries under a load of up to 200 concurrent users."
"Reliable" → "The system shall have an availability of at least 99.9% measured over a rolling 30-day window" (or, alternatively, "mean time between failures shall exceed 5,000 hours with a mean time to repair of no more than 5 hours").
"User-friendly" → "A first-time user, given no training, shall be able to complete a standard order-entry task within 5 minutes with no more than one recoverable error."
Quantification serves two purposes beyond testability: it lets NFRs be objectively verified during acceptance testing exactly like functional requirements, and it lets NFRs be explicitly TRADED OFF against cost and schedule — a stakeholder can be shown, concretely, what raising availability from 99.9% to 99.99% (10x fewer permitted outage-minutes per year — about 526 minutes down to about 53) would cost in redundant infrastructure, rather than negotiating over an undefined word like "reliable."
