Introduction
The difference between good QA engineers and great QA engineers often comes down to the quality of their test cases. A well-designed test case can catch bugs before they reach production. A poorly designed one might miss critical issues or waste time testing irrelevant scenarios. In this article, I'll share techniques and best practices for writing test cases that actually find bugs.
Characteristics of Effective Test Cases
- Clear and concise: Anyone should understand what's being tested without ambiguity
- Independent: Test cases should not depend on each other
- Repeatable: Same result every time they're executed
- Traceable: Linked to requirements for traceability
- Complete: Include all necessary preconditions, steps, and expected results
- Maintainable: Easy to update as requirements change
Test Case Design Techniques
1. Boundary Value Analysis (BVA)
Most bugs occur at boundaries. Test at and around limit values.
Test cases should include:
• Age = 17 (below minimum)
• Age = 18 (minimum boundary)
• Age = 19 (just above minimum)
• Age = 64 (just below maximum)
• Age = 65 (maximum boundary)
• Age = 66 (above maximum)
2. Equivalence Partitioning
Divide input into groups (partitions) where all inputs behave identically. Test one value from each partition.
Partitions:
• Valid emails: user@example.com
• Missing @: userexample.com
• Multiple @: user@@example.com
• Missing domain: user@
Instead of testing 100 valid emails, test just 1 from each partition.
3. Decision Tables
When multiple conditions affect behavior, use decision tables to ensure all combinations are tested.
Conditions: Income ≥ $50K, Credit Score ≥ 700, Employment ≥ 2 years
| Income | Credit | Employment | Result |
|---|---|---|---|
| Y | Y | Y | Approve |
| Y | N | Y | Reject |
| N | N | N | Reject |
4. Error Guessing
Use experience and domain knowledge to guess where errors might occur. This is less formal but often catches real bugs.
- Negative inputs when positives are expected
- Zero and null values
- Very large inputs (overflow)
- Empty fields
- Special characters in text fields
- Duplicate data
Test Case Structure
Every test case should follow a consistent structure:
Title: Verify successful login with valid credentials
Preconditions: User account exists with username "testuser" and password "Test@123"
Test Steps:
1. Navigate to login page
2. Enter username: testuser
3. Enter password: Test@123
4. Click Login button
Expected Result: User is redirected to dashboard with welcome message
Severity: Critical
Priority: High
Test Coverage Considerations
Balance Quality vs. Quantity
Don't try to test everything. Focus on:
- Critical functionality that users rely on
- Features that handle sensitive data
- Complex business logic
- Integration points between systems
- Areas with high change frequency
Prioritize Test Cases
- P0: Critical - system doesn't work without these
- P1: High - major functionality affected
- P2: Medium - minor features affected
- P3: Low - nice to have, cosmetic issues
Maintaining Test Cases Over Time
Version Control
Keep test cases in version control alongside code. Document changes and reasons.
Regular Review
Review test cases quarterly. Remove duplicate tests, consolidate similar ones, and update for new features.
Traceability Matrix
Maintain a matrix linking test cases to requirements. This ensures you have coverage for every requirement and helps identify untested features.
Common Mistakes to Avoid
- Over-testing trivial scenarios: Focus on complex and risky areas
- Dependent test cases: Each test should run independently
- Unclear expected results: Make them specific and measurable
- Ignoring edge cases: That's where bugs hide
- No traceability: Always link tests to requirements
- Outdated test cases: Keep them current with application changes
Conclusion
Writing effective test cases is an art and a science. By combining systematic techniques like boundary value analysis with experience-based error guessing, you'll create test suites that actually find bugs. Remember: the goal isn't to test everything, but to test the right things in the right way.
Start with well-structured, clear test cases. Use design techniques to identify critical scenarios. Maintain and update your test suite as the application evolves. Do this consistently, and you'll become the QA engineer developers trust and respect!