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Cost of Defects

1. Definition

Cost of Defects refers to the increase in time, effort, money, and business impact required to fix a defect, depending on when it is discovered in the software development lifecycle.

  • The later a defect is found, the higher the cost to fix it.

2. Why Cost of Defects Is Important

  • Helps justify early testing
  • Shows business value of manual testing
  • Explains why requirement reviews matter
  • Supports risk-based testing decisions

3. Defect Cost Across SDLC Phases

Phase Found Cost Impact Reason
Requirement Phase Very Low Simple clarification
Design Phase Low Minor redesign
Development Phase Medium Code changes + rework
Testing Phase High Code + retesting
Production Very High Hotfixes, downtime, reputation loss

4. Cost Components of a Defect

A defect may incur:

  • Developer rework cost
  • Tester rework cost
  • Delay in release
  • Customer support cost
  • Production outage cost
  • Brand and trust damage

5. Real-Time Example

Password rule missing in requirement:

  • Found during requirement review → Quick fix
  • Found during testing → Code + test update
  • Found in production → User complaints + hotfix + support

6. Defect Leakage and Cost

  • Defect Leakage: Defects escaping to next phase or production
  • Higher leakage → Higher cost
  • Testing aims to minimize leakage

7. Role of Manual Testing in Reducing Cost

Manual testers reduce defect cost by:

  • Reviewing requirements early
  • Asking clarifying questions
  • Designing effective test cases
  • Executing negative and edge scenarios
  • Supporting UAT

8. Cost of Defects vs Time Pressure

  • Skipping testing may save time initially
  • Leads to higher cost later
  • “Fix later” is always more expensive

9. Common Misconceptions

  • Fewer defects ≠ lower cost
  • Minor defects in production can still cause major impact
  • Testing is not a cost center—it is cost prevention

10. Interview-Ready Answers

Short answer:

Cost of defects refers to the increasing effort, time, and money required to fix a defect depending on the phase in which it is detected.

Detailed answer:

The cost of defects increases significantly when defects are detected late in the lifecycle, especially in production, due to rework, downtime, and business impact.

11. Key Takeaway

Early detection through verification and manual testing is the most effective way to control defect cost.