# Test Case Design

> Design comprehensive test cases using boundary analysis, equivalence partitioning, decision tables, and state transition testing. Use when creating test plans, designing test coverage, or when user asks about testing strategies.

- Skill: `testified-oss/test-case-design` (Agent Skill)
- Install (CLI): `npx skillmds@latest add testified-oss/test-case-design`
- Raw SKILL.md: https://api.skillmd.com/api/skills/testified-oss/test-case-design/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: testified-oss (https://skillmd.com/u/testified-oss)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/testified-oss/test-case-design

---


# Test Case Design

## When to Use

- Creating test plans or test suites
- Designing comprehensive test coverage
- Reviewing existing test coverage for gaps
- When user asks about testing strategies
- Identifying edge cases and boundary conditions
- Testing complex business logic with multiple conditions

## When NOT to Use

- Simple unit tests with obvious inputs
- When user already has specific test cases defined
- Performance or load testing (different techniques)
- Security penetration testing

## Test Design Techniques

### 1. Equivalence Partitioning

Divide inputs into groups (partitions) where all values in a partition should behave identically.

#### Process

1. Identify input parameters
2. Divide each parameter into valid and invalid partitions
3. Select one representative value from each partition
4. Create test cases covering each partition

#### Example

For a function `validateAge(age)` where valid age is 18-65:

| Partition | Range | Representative Value | Expected Result |
|-----------|-------|---------------------|-----------------|
| Invalid (too low) | < 18 | 10 | Invalid |
| Valid | 18-65 | 35 | Valid |
| Invalid (too high) | > 65 | 80 | Invalid |
| Invalid (negative) | < 0 | -5 | Invalid |
| Invalid (non-numeric) | N/A | "abc" | Invalid |

### 2. Boundary Value Analysis

Test at the edges of equivalence partitions where defects often occur.

#### Process

1. Identify boundaries of each partition
2. Test at: minimum, minimum-1, minimum+1, maximum-1, maximum, maximum+1
3. Include special values: 0, empty, null

#### Example

For `validateAge(age)` with valid range 18-65:

| Boundary | Value | Expected Result |
|----------|-------|-----------------|
| Below minimum | 17 | Invalid |
| At minimum | 18 | Valid |
| Above minimum | 19 | Valid |
| Below maximum | 64 | Valid |
| At maximum | 65 | Valid |
| Above maximum | 66 | Invalid |
| Zero | 0 | Invalid |
| Null | null | Invalid |

### 3. Decision Tables

Test combinations of conditions and their resulting actions. Essential for complex business rules.

#### Process

1. List all conditions (inputs)
2. List all possible actions (outputs)
3. Create columns for each condition combination
4. Mark which actions occur for each combination

#### Example

Shipping discount rules:
- Condition 1: Order > $100
- Condition 2: Premium member
- Condition 3: Holiday sale active

| Rule | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|------|---|---|---|---|---|---|---|---|
| **Conditions** |
| Order > $100 | Y | Y | Y | Y | N | N | N | N |
| Premium member | Y | Y | N | N | Y | Y | N | N |
| Holiday sale | Y | N | Y | N | Y | N | Y | N |
| **Actions** |
| 30% discount | X | | | | | | | |
| 20% discount | | X | X | | X | | | |
| 10% discount | | | | X | | X | X | |
| No discount | | | | | | | | X |

#### Decision Table Template

```markdown
| Rule | 1 | 2 | 3 | 4 |
|------|---|---|---|---|
| **Conditions** |
| Condition A | Y | Y | N | N |
| Condition B | Y | N | Y | N |
| **Actions** |
| Action 1 | X | | | |
| Action 2 | | X | X | |
| Action 3 | | | | X |
```

### 4. State Transition Testing

Test how a system moves between different states based on events.

#### Process

1. Identify all possible states
2. Identify events that trigger transitions
3. Create state transition diagram
4. Derive test cases to cover all transitions

#### Example

Order Status System:

```
States: [Created] -> [Confirmed] -> [Shipped] -> [Delivered]
                  \-> [Cancelled]
```

| Test | Start State | Event | Expected End State |
|------|-------------|-------|-------------------|
| TC1 | Created | confirm() | Confirmed |
| TC2 | Created | cancel() | Cancelled |
| TC3 | Confirmed | ship() | Shipped |
| TC4 | Confirmed | cancel() | Cancelled |
| TC5 | Shipped | deliver() | Delivered |
| TC6 | Shipped | cancel() | Error (cannot cancel) |
| TC7 | Delivered | cancel() | Error (cannot cancel) |
| TC8 | Cancelled | confirm() | Error (cannot confirm) |

#### State Transition Matrix

| Current State | confirm() | cancel() | ship() | deliver() |
|--------------|-----------|----------|--------|-----------|
| Created | Confirmed | Cancelled | - | - |
| Confirmed | - | Cancelled | Shipped | - |
| Shipped | - | Error | - | Delivered |
| Delivered | - | Error | - | - |
| Cancelled | Error | - | - | - |

## Test Case Template

Use this format for documenting test cases:

```markdown
### TC-[ID]: [Brief Description]

**Technique:** [Boundary/Equivalence/Decision/State]
**Priority:** [High/Medium/Low]
**Preconditions:**
- [Required state before test]

**Test Steps:**
1. [Step 1]
2. [Step 2]

**Test Data:**
- Input: [values]

**Expected Result:**
- [What should happen]

**Actual Result:**
- [Fill in during execution]

**Status:** [Pass/Fail/Blocked]
```

## Coverage Guidelines

### Minimum Coverage

| Technique | Minimum Requirement |
|-----------|-------------------|
| Equivalence Partitioning | All partitions covered |
| Boundary Analysis | All boundaries tested |
| Decision Tables | All rule combinations |
| State Transition | All transitions covered |

### Comprehensive Coverage

- **0-switch coverage**: Cover all valid single transitions
- **1-switch coverage**: Cover all valid pairs of consecutive transitions
- **Invalid transitions**: Test that invalid transitions are properly rejected

## Combining Techniques

For thorough testing, combine techniques:

1. **Start with Equivalence Partitioning** to identify input groups
2. **Apply Boundary Analysis** to partition boundaries
3. **Use Decision Tables** for complex condition combinations
4. **Add State Transition** for stateful components
5. **Review for missing edge cases**

## Output Format

When designing test cases, provide:

```markdown
## Test Design for [Feature Name]

### Analysis

**Input Parameters:**
- Parameter 1: [type, constraints]
- Parameter 2: [type, constraints]

**Equivalence Partitions:**
| Parameter | Partition | Values | Valid/Invalid |
|-----------|-----------|--------|---------------|
| ... | ... | ... | ... |

**Boundaries:**
| Parameter | Boundary | Test Values |
|-----------|----------|-------------|
| ... | ... | ... |

### Test Cases

[Use test case template for each case]

### Coverage Summary

- Total test cases: X
- Equivalence partitions covered: X/Y
- Boundaries tested: X/Y
- Decision rules covered: X/Y
- State transitions covered: X/Y
```

## Best Practices

1. **Prioritize** - Focus on high-risk areas first
2. **Be systematic** - Don't rely on intuition alone
3. **Document assumptions** - Note what's not tested and why
4. **Review with stakeholders** - Validate test coverage
5. **Automate where possible** - Parameterized tests for partitions/boundaries
6. **Maintain traceability** - Link test cases to requirements