Why Quality Requirements Should Be Quantifiable: 25-Comp-B11 Advanced Software Design, December 2018, Q3

Question 3 of 28: Why Quality Requirements Should Be Quantifiable

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

17-Comp-B11 Advanced Software Design — National Exams, December 2018. 3 hours, closed book exam with up to 2 aid sheets allowed (written on both sides), no calculator permitted. The paper is organized into five parts, and candidates were instructed to answer any five (5) questions in Part I, any three (3) in Part II, any four (4) in Part III, any two (2) in Part IV, and any five (5) 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 19 questions actually marked (5+3+4+2+5 of 28) each count for 100/19 ≈ 5.26% of the paper. All 28 questions are answered below for completeness.

Reference texts: Sommerville, Software Engineering (10th ed., Pearson) — software processes, requirements engineering, 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 — creational/structural/behavioural pattern catalogue and the "program to an interface, not an implementation" / "favor object composition over class inheritance" principles; 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; Barbara Liskov's 1987 substitutability paper for Question 11; Karl Wiegers, Software Requirements (3rd ed.) — the functional/quality/process/implementation/business requirements taxonomy of Question 2; Kruchten, The Rational Unified Process: An Introduction, for Question 1; Myers, The Art of Software Testing, for Questions 6 and 28.

PART I — General Principles (answer any 5 of 7)

Question 3: Why Quality Requirements Should Be Quantifiable (Part I)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A quality requirement (Question 2b) 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," "reliable," or "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."
"User-friendly" → "A first-time user, given no training, shall complete a standard order-entry task within 5 minutes with no more than one recoverable error."

Quantification serves two purposes beyond testability: it lets quality requirements be objectively verified during acceptance testing exactly like functional requirements, and it lets them be explicitly TRADED OFF against cost and schedule — a stakeholder can be shown, concretely, what raising availability from 99.9% to 99.99% would cost in redundant infrastructure, rather than negotiating over an undefined word like "reliable."