Cost of Quality — Revision Questions

Purpose: Self-study and exam preparation Format: Explain in your own words, give brief examples

Question 1: Why Cost of Quality Matters

Why is it important to express software quality issues in financial terms rather than technical metrics?

Give an example of how you would reframe a technical quality problem (e.g., “fault density increased”) into business language for management.

Question 2: The Four CoQ Categories

The Cost of Quality framework divides costs into four categories: Prevention, Appraisal, Internal Failure, and External Failure.

Explain the difference between conformance costs and nonconformance costs. For each of the four categories, give one concrete example from a software development project.

Question 3: Prevention vs Appraisal

Weinberg observed: “When managers say ‘Testing takes too long,’ what they should be saying is ‘Fixing the bugs takes too long.’”

Explain the difference between appraisal costs and internal failure costs using this insight. Why is this distinction important for quality planning?

Question 4: The 1:10:100 Rule

The 1:10:100 rule states that fixing a defect costs $1 in requirements, $10 in development, and $100+ after release.

Explain why this cost escalation occurs. How does “shift-left” testing leverage this principle to reduce total CoQ?

Question 5: Traditional vs Modern CoQ Models

The traditional CoQ model (1950s-1970s) suggested there is an optimal quality level below 100% conformance. The modern model (Kondo, Juran) argues the optimum can extend toward 100%.

What is the key difference in assumptions between these models? What does Knox mean by “do something different”?

Question 6: Process Maturity and CoQ

Raytheon’s empirical data showed that moving from CMM Level 1 to Level 3 reduced total CoSQ from ~65% to ~20% of project costs.

Which CoQ category showed the largest reduction? Explain why process improvement has this effect on the cost structure.

Question 7: Tank and Pipes Model

The “tank and pipes” model visualizes defects flowing into a system (injection) and being removed through V&V activities (removal).

Draw or describe this model and explain what “residual defects” represent. Why is it important to track both injection and removal rates?

Question 8: Improvement Strategies

You have two options to reduce field defects:

Option A: Improve unit testing effectiveness from 50% to 70%
Option B: Add code inspections that catch 20% of defects before testing

Both achieve the same residual defect count. Which option is more cost-effective and why?

Question 9: Making CoQ Actionable

To implement a CoQ program, you need data from multiple sources.

List three data sources for CoQ measurement and explain what quality cost information each provides. Why is defect classification (origin phase, detection phase, type) essential for improvement?

Question 10: Technical Debt and CoQ

Technical debt can be viewed as a form of deferred quality cost.

Explain how technical debt relates to the CoQ framework. What are the implications of quantifying technical debt as an increase in the “cost of change”?

Topics Covered

Topic	Questions
Business case for CoQ	Q1
Four CoQ categories (PAF)	Q2, Q3
Economics of quality	Q4, Q5
Process maturity impact	Q6
Defect injection/removal	Q7, Q8
CoQ implementation	Q9
Technical debt	Q10