Question 3 of 27: 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 2015. 3 hours, closed book exam with one aid sheet allowed (written on both sides), no calculator permitted. The paper is organized into five parts, and candidates were instructed to answer any four (4) questions in Part I, any three (3) in Part II, any three (3) in Part III, any two (2) in Part IV, and any four (4) 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 16 questions actually marked (4+3+3+2+4 of 27) each count for 100/16 ≈ 6.25% of the paper. All 27 questions are answered below for completeness.
Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, agile methods, design principles, dependability; 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 (Adapter, Bridge, Strategy, Observer, Template Method, Composite, etc.); Sebesta, Concepts of Programming Languages (12th ed.) — polymorphism, dynamic binding, inheritance and language-level object semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/invariants, the open–closed principle; Barbara Liskov's 1987 substitutability paper for Question 11; Rogers, Sharp & Preece, Interaction Design, and Nielsen, Usability Engineering, for Question 21's HMI-specific non-functional requirements.
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" (equivalently, "mean time between failures shall exceed 5,000 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% (ten times fewer permitted outage-minutes: about 526 minutes per year down to about 53) would cost in redundant infrastructure, rather than negotiating over an undefined word like "reliable."
