Test Case Design Technique: Equivalence Partitioning
Overview
Equivalence Partitioning (EP) is a black-box test design technique that divides input data into partitions or classes where the system is expected to behave similarly. By identifying these equivalence classes, testers can reduce the number of test cases while maintaining effective coverage.
Instead of testing every possible input, EP helps choose representative values from each class, assuming if one value in the class works, the rest will too.
Why Use Equivalence Partitioning?
- Reduces redundancy in test cases
- Improves test efficiency by covering more logic with fewer tests
- Focuses on valid and invalid input handling
- Enhances clarity in requirement-based testing
How It Works
- Analyze input fields or parameters.
- Identify sets of valid and invalid data.
- Group data into equivalence classes:
- Valid classes: Inputs that should be accepted
- Invalid classes: Inputs that should be rejected
- Select one representative value from each class to test.
Simple Example: Age Input Field
Imagine a form that accepts user age between 18 and 65 inclusive.
Step 1: Identify Equivalence Classes
| Class Type | Range | Description |
|---|---|---|
| Valid | 18 to 65 | Acceptable age range |
| Invalid | < 18 | Too young |
| Invalid | > 65 | Too old |
| Invalid | Non-numeric | e.g., "abc", "!@#" |
Step 2: Choose Test Inputs
You only need to test one representative value from each class.
| Class | Representative Value | Expected Result |
|---|---|---|
| Valid | 30 | Accepted |
| Invalid (<18) | 17 | Rejected - too young |
| Invalid (>65) | 70 | Rejected - too old |
| Invalid (text) | "abc" | Rejected - not a number |
Real-World Example: Password Length Validation
Requirement: Password must be between 8 and 16 characters.
| Class Type | Range | Example Value | Expected Result |
|---|---|---|---|
| Valid | 8–16 chars | "Password123" | Accepted |
| Invalid | < 8 chars | "Pass1" | Rejected |
| Invalid | > 16 chars | "VeryLongPassword!" | Rejected |
By selecting only one value per class, you ensure broad functional coverage with minimal test cases.
Combining with Other Techniques
Equivalence Partitioning is often combined with:
- Boundary Value Analysis (BVA) for edge conditions
- Decision Table Testing for complex rules
- State Transition Testing for dynamic systems
Best Practices
- Clearly define the valid/invalid criteria from requirements.
- Document each equivalence class and rationale.
- Cover at least one test per partition to ensure logic is verified.
- Use EP early in test design to save effort later.
Summary
Equivalence Partitioning helps testers focus on the most meaningful test inputs by dividing data into logical groups. It reduces test suite size without compromising quality and is especially useful for input validation, form fields, and API parameters.
"Don’t test everything. Test smartly. Equivalence Partitioning is about maximum coverage with minimum effort."