Testing principles, psychology, errors vs defects vs failures
Domain Weight
Fundamentals of Testing accounts for 15% of the CTFL exam.
CTFL scenarios test whether you understand why testing exists and how testers collaborate — not just glossary definitions.
| Term | Definition |
|---|
| Error | Human mistake that produces incorrect result |
| Defect (Bug) | Flaw in work product — code, spec, doc |
| Failure | Deviation of actual from expected during execution |
| Testing | Evaluating work products to find defects and reduce risk |
Seven Testing Principles
- Testing shows presence of defects, not their absence
- Exhaustive testing is impossible — use risk-based prioritization
- Early testing saves cost — shift-left in SDLC
- Defect clustering — few modules contain most bugs
- Pesticide paradox — repeating same tests stops finding new defects
- Testing is context-dependent — safety-critical vs marketing site differ
- Absence-of-errors fallacy — bug-free product may still fail user needs
Psychology of Testing
Independent testing brings objectivity. Testers report facts without blame. Developers and testers share quality goals but different perspectives.
Exam Focus Areas
- Distinguish error (human) → defect (product) → failure (runtime)
- Independence levels: developer tests own code < peer review < independent team
- Test objectives include finding defects, building confidence, preventing defects
Practice This Domain
Test your understanding with free practice questions at /certifications/istqb/ctfl/practice — focus on: Why testing is necessary, Test objectives, Testing principles.
