Software Metrics and the Meaning of "Quantifiable": 25-Comp-B11 Advanced Software Design, December 2019, Q3

Question 3 of 28: Software Metrics and the Meaning of "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 2019. 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 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, testing, 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, visibility, and multiple inheritance semantics; Bertrand Meyer, Object-Oriented Software Construction — design by contract, preconditions/postconditions/class invariants; Barbara Liskov's 1987 substitutability paper for Question 12; Karl Wiegers, Software Requirements (3rd ed.); Myers, The Art of Software Testing, for Question 6.

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

Question 3: Software Metrics and the Meaning of "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 software metric is a quantitative measure of some attribute of a software product, process, or project — for example lines of code, cyclomatic complexity, coupling between objects (CBO), defect density, or mean time between failures. Metrics turn a qualitative notion ("this module is complex," "this design is loosely coupled") into a number that can be recorded, compared against a threshold or an earlier version, and tracked over time.

"Quantifiable" and "software metrics" are two sides of the same relationship: a quality attribute is quantifiable precisely when a metric exists that measures it. Without a metric, "high-quality" is an unmeasurable adjective — two designers can disagree about whether a design is high-quality with no way to settle the disagreement. With a metric (e.g. cyclomatic complexity for testability, or CBO for modularity), a numeric target can be set (e.g. "cyclomatic complexity ≤ 10 per method"), design alternatives can be objectively compared against it, and progress toward the quality goal can be tracked as the design evolves. Software metrics are therefore the mechanism that makes a quality concern (Question 5) into a specific, verifiable design constraint rather than a vague aspiration.