Testing Principles

Learning objectives#

• List the seven testing principles
• Apply them to real decisions
• Explain the pesticide paradox
• Explain the absence-of-errors fallacy

Explanation#

1) Testing shows the presence of defects, not their absence. 2) Exhaustive testing is impossible — prioritise by risk.

3) Early testing saves time and money. 4) Defects cluster together — a few modules contain most bugs.

5) Pesticide paradox — repeating the same tests stops finding new bugs; refresh them.

6) Testing is context dependent — a game and a medical device need different testing. 7) Absence-of-errors fallacy — bug-free software that does not meet user needs is still a failure.

Code example: Why exhaustive testing is impossible#

1Login form: username (up to 30 chars), password (8–20 chars), "Remember me" checkbox
2

3Possible usernames ≈ 95^30   (printable characters, every length)
4Possible passwords ≈ 95^20
5× 2 checkbox states
6→ far more combinations than could ever be executed
7

8So we choose: valid login, wrong password, empty fields, boundary lengths,
9special characters, SQL-injection text, locked account, remember-me on/off.

A few dozen well-chosen tests cover the real risks instead of trillions of combinations.

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. Count the combinations to see they are effectively infinite.
2. Use risk and techniques to choose representative tests.
3. Focus on boundaries, errors and security.
4. Refresh tests as the product changes.

Real-world use case#

Common mistakes#

Best practices#

• Prioritise by risk
• Track where defects cluster
• Review and refresh test suites
• Adapt the approach to the domain

Remember this#

Practice exercise#

1Defect clustering.

Mini quiz#

Want more? Practise all Manual Testing MCQs · Manual Testing interview questions

Related topics#

My notes#

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