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CTFL 1.3.1: Explain the seven testing principles

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Syllabus section 1.3

4 original PrepBench practice questions for learning objective 1.3.1 of the CTFL 4.0 syllabus. Try the examples below and check each answer against its syllabus reference.

Practice questions

Question 1

A test team has run the same regression suite unchanged for two years. Recently the suite finds almost no new defects, although users still report problems in production. Which testing principle BEST explains this situation?

  1. Exhaustive testing is impossible
  2. Defects cluster together
  3. Tests wear out
  4. Absence-of-defects fallacy
Show answer
  • Exhaustive testing is impossibleThis principle is about the infeasibility of testing everything, not tests losing effectiveness over time.
  • Defects cluster togetherClustering concerns where defects concentrate, not why repeated tests stop finding new ones.
  • Correct answer: Tests wear outRepeating the same tests unchanged eventually stops revealing new defects — the tests wear out.
  • Absence-of-defects fallacyThat fallacy is about meeting user needs, not about a suite losing its defect-finding power.

The principle 'tests wear out' (formerly the pesticide paradox) states that if the same tests are repeated over and over, they eventually stop finding new defects. Existing tests and test data need to be modified, and new tests written, to keep finding defects.

Syllabus section 1.3

Question 2

An analysis of a defect report database shows that 70% of all defects were found in 15% of the system's modules. Which testing principle does this observation illustrate?

  1. Defects cluster together
  2. Testing is context dependent
  3. Early testing saves time and money
  4. Testing shows the presence, not the absence of defects
Show answer
  • Correct answer: Defects cluster togetherA few modules containing most of the defects is exactly the defect clustering principle.
  • Testing is context dependentContext dependence is about tailoring testing to context, not the concentration of defects.
  • Early testing saves time and moneyEarly testing concerns when to test, not where defects are concentrated.
  • Testing shows the presence, not the absence of defectsThis principle is about what testing can prove, not the distribution of defects across modules.

Defect clustering means a small number of components usually contain most of the defects found. Such clusters are a valuable input for risk-based testing, which focuses test effort on those areas.

Syllabus section 1.3

Question 3

A project delivered software that passed all of its tests, yet users rejected it because it did not fit their working procedures. Which testing principle BEST describes what the project ignored?

  1. Tests wear out
  2. Absence-of-defects fallacy
  3. Defects cluster together
  4. Exhaustive testing is impossible
Show answer
  • Tests wear outWearing out is about repeated tests; here the issue is software that does not meet user needs.
  • Correct answer: Absence-of-defects fallacyFinding few defects does not guarantee success — the system must also meet users' needs.
  • Defects cluster togetherClustering concerns where defects are, not whether the validated system fits user needs.
  • Exhaustive testing is impossibleInfeasibility of exhaustive testing is unrelated to software failing to meet user expectations.

The absence-of-defects fallacy: it is a fallacy to expect that verifying the software and finding few defects guarantees success. The system must also meet users' needs and expectations — validation matters as well as verification.

Syllabus section 1.3

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