Error Guessing: A Complete Guide for ISTQB CTFL Exam

Error Guessing: A Complete Guide for ISTQB CTFL Exam

What is Error Guessing?

Error guessing is a software testing technique that relies on the intuition, experience, and domain knowledge of testers to identify potential errors and defects that might not be detected by formal test design methods. Unlike systematic testing techniques that follow predefined rules and structures, error guessing is an informal, experience-based approach where testers use their judgment to anticipate where bugs are likely to occur.

In essence, error guessing asks: "What could go wrong here based on what I know about similar systems, user behavior, and common programming mistakes?"

Why is Error Guessing Important?

1. Catches Unexpected Defects
Error guessing complements formal testing techniques by identifying defects that structured methods might miss. Experienced testers can spot potential issues that test cases based solely on requirements might overlook.

2. Cost-Effective Testing
Rather than creating exhaustive test cases for every scenario, testers can quickly identify high-risk areas and focus their testing efforts there, making the testing process more efficient.

3. Leverages Tester Experience
It utilizes the knowledge and intuition of experienced testers who have seen similar issues in the past. This accumulated experience is valuable and difficult to replicate through formal techniques alone.

4. Addresses Domain-Specific Risks
Different industries and applications have different failure patterns. Error guessing allows testers to apply domain-specific knowledge to anticipate failures unique to their field.

5. Handles Unclear Requirements
When requirements are ambiguous or incomplete, error guessing helps testers identify potential problem areas without waiting for clarification.

6. Improves Risk-Based Testing
By identifying high-risk areas, error guessing supports risk-based testing strategies that allocate resources to the most critical areas.

How Does Error Guessing Work?

Foundation: Experience and Knowledge
Error guessing is built on the premise that experienced testers have seen common failure patterns. These patterns come from:

• Previous testing experience on similar systems
• Knowledge of common programming errors
• Understanding of user behavior and expectations
• Familiarity with the domain (banking, healthcare, e-commerce, etc.)
• Knowledge of the specific technology and platform being tested

The Error Guessing Process

Step 1: Analyze the System
Understand what the software does, how it works, and what users expect from it. Review requirements, design documents, and existing code if available.

Step 2: Identify High-Risk Areas
Based on experience, identify components or features most likely to have defects. These might include:

• Complex algorithms or calculations
• Areas requiring integration between components
• Boundary conditions and edge cases
• Error handling and exception management
• Security-sensitive functionality
• Areas that have historically had defects

Step 3: Brainstorm Potential Failures
Use intuition and experience to imagine what could go wrong. Consider:

• What assumptions are being made?
• What could users do unexpectedly?
• What are common programming mistakes?
• What boundary conditions exist?
• What happens when something fails?

Step 4: Design Test Cases
Create informal test cases targeting identified potential failures. These tests don't follow strict methodologies but are guided by the guesses about what might break.

Step 5: Execute and Document
Run the tests and document any findings. Even if the guessed defects aren't found, the testing still provides value through coverage of risky areas.

Common Error Guessing Categories

Boundary and Limit Errors
Guessing that problems occur at boundaries: entering 0, negative numbers, maximum values, or minimum values in numeric fields.

Input Validation Errors
Assuming the system might not properly validate input: special characters, empty fields, very long strings, or unexpected data types.

Calculation Errors
Guessing mathematical operations might fail: rounding errors, division by zero, overflow conditions, or incorrect formulas.

Error Handling Defects
Assuming error messages might be missing or unhelpful, or that error conditions might not be handled gracefully.

State-Related Errors
Guessing problems when transitioning between states or when operations are performed in unexpected sequences.

Integration Errors
Assuming components might not communicate properly: missing data, format mismatches, or timing issues.

Performance Issues
Guessing that systems might slow down with large datasets, concurrent users, or complex operations.

Security Vulnerabilities
Assuming common security flaws: SQL injection, cross-site scripting, insufficient authentication, or privilege escalation.

Error Guessing vs. Other Test Design Techniques

Strengths and Limitations of Error Guessing

Strengths:

• Catches defects that formal techniques miss
• Efficient and quick to apply
• Leverages valuable tester experience
• Flexible and adaptable
• Cost-effective for resource allocation
• Useful when requirements are unclear

Limitations:

• Highly dependent on tester experience and skill
• Results vary significantly between testers
• Not repeatable or easily documented
• Can lead to inconsistent coverage
• Difficult to measure effectiveness
• May miss systematic patterns that formal techniques would catch
• Not suitable as the only testing approach

Best Practices for Effective Error Guessing

1. Combine with Formal Techniques
Use error guessing as a complement to systematic testing methods like equivalence partitioning and boundary value analysis, not as a replacement.

2. Document Your Guesses
Write down the reasoning behind your guesses. This helps other testers understand your thinking and creates a knowledge base for future testing.

3. Leverage Team Experience
Conduct brainstorming sessions where multiple testers contribute their experience. Different perspectives increase the likelihood of identifying defects.

4. Build on Lessons Learned
Review past defects and root causes to inform future error guessing. If a similar issue occurred before, it's likely to occur again in similar situations.

5. Focus on High-Risk Areas
Don't guess randomly. Target areas most likely to have defects based on complexity, criticality, or historical data.

6. Consider the User Perspective
Think about how real users might interact with the system and what mistakes they might make.

7. Use Checklists
Create and maintain checklists of common errors in your domain to guide error guessing and ensure consistency.

How to Answer Exam Questions on Error Guessing

Understanding Question Types

ISTQB CTFL exam questions on error guessing typically fall into these categories:

1. Definitional Questions
These ask what error guessing is, what it's based on, or how it's defined.

Example: "Error guessing is based primarily on..."

Answer Focus: Error guessing is based on experience and intuition of the tester. Key terms include informal, unstructured, depends on tester experience, and anticipating potential defects.

2. Characteristics Questions
These ask about the nature, strengths, or limitations of error guessing.

Example: "Which of the following is a characteristic of error guessing?"

Answer Focus: Look for keywords like informal, experience-based, unstructured, intuition, not repeatable, or not systematic. Avoid answers suggesting systematic procedures or formal rules.

3. Application Questions
These present scenarios and ask how error guessing would be used.

Example: "In which situation would error guessing be most useful?"

Answer Focus: Error guessing is most useful when:

• Requirements are unclear or ambiguous
• Testing time is limited
• You need to identify high-risk areas quickly
• Using it to supplement formal testing techniques
• Domain-specific knowledge is critical
• You have experienced testers available

4. Comparison Questions
These compare error guessing to other test design techniques.

Example: "How does error guessing differ from equivalence partitioning?"

Answer Focus: Key differences:

• Error guessing is informal; equivalence partitioning is systematic
• Error guessing is not repeatable; equivalence partitioning is
• Error guessing depends on experience; equivalence partitioning follows defined rules
• Error guessing is unstructured; equivalence partitioning is structured

5. When to Use Questions
These ask about appropriate contexts for applying error guessing.

Example: "When should error guessing be employed?"

Answer Focus: Error guessing should be used:

• To find defects that formal techniques might miss
• As a supplement to other testing techniques
• When time and resources are limited
• For high-risk areas identified through risk analysis
• When testers have relevant experience

Exam Tips: Answering Questions on Error Guessing

Tip 1: Remember the Core Definition
Error guessing = Informal + Experience-Based + Intuition-Driven
Any answer emphasizing these aspects is likely correct. Avoid answers suggesting systematic or formal procedures.

Tip 2: It's a Complement, Not a Replacement
When answering questions about when to use error guessing, remember it should supplement formal testing techniques, not replace them. If an answer suggests error guessing alone is sufficient, it's likely wrong.

Tip 3: Recognize "Unstructured" and "Non-Repeatable" as Key Characteristics
Error guessing is explicitly non-systematic and non-repeatable. If a question asks what makes error guessing different from other techniques, these characteristics often distinguish it. If an answer suggests error guessing is structured or repeatable, it's wrong.

Tip 4: Experience and Domain Knowledge Matter
Many questions emphasize that error guessing depends on tester experience, knowledge, and intuition. If an answer option emphasizes the importance of the tester's background and expertise, it's likely correct.

Tip 5: Look for Risk and High-Risk Areas
Error guessing is often applied to high-risk areas or when you need to quickly identify potential defects. Answers mentioning risk, critical areas, or rapid defect detection are often correct.

Tip 6: Watch for "All Testing Should Use Error Guessing"
Be cautious of absolute statements. Error guessing is valuable but should be combined with other techniques. Answers suggesting it should be the primary or only approach are likely incorrect.

Tip 7: Understand Common Error Categories
Questions might ask which types of errors error guessing is particularly good at finding. The answer often includes:

• Boundary and limit errors
• Input validation errors
• State-related errors
• Integration errors
• Unusual or unexpected scenarios

Tip 8: Distinguish from Formal Techniques
When comparing error guessing to equivalence partitioning, boundary value analysis, or decision table testing, remember:

• Those techniques are formal and systematic
• Error guessing is informal and intuitive
• Those techniques are repeatable
• Error guessing is not repeatable

If you're unsure which technique an answer describes, these distinctions help eliminate wrong options.

Tip 9: Apply Real-World Thinking
Error guessing questions often have practical elements. Think about:

• How would an experienced tester approach this?
• What would they likely try first?
• What would they focus on based on their experience?

This practical thinking helps you answer application and scenario-based questions.

Tip 10: Read Carefully for "Best" or "Most Appropriate"
Questions often ask what is the "best use" or "most appropriate application" of error guessing. The answer is usually about:

• Using it with other techniques
• Focusing on high-risk areas
• Leveraging experienced testers
• Supplementing formal methods

Avoid answers suggesting error guessing alone is sufficient or is the primary testing approach.

Tip 11: Know When NOT to Use Error Guessing
Sometimes questions ask when error guessing is not suitable. The answer typically involves:

• When formal, repeatable, and documented approaches are required
• When you don't have experienced testers
• When you need comprehensive and systematic coverage
• When results must be reproducible and auditable

Tip 12: Practice with Examples
If given a scenario, try to identify:

• Would error guessing be useful here?
• What would an experienced tester guess might break?
• Is this a high-risk area?
• Should error guessing be combined with other techniques?

This practice helps you apply the concept rather than just memorize definitions.

Sample Exam Questions and Explanations

Question 1: "Which statement best describes error guessing?"

A) A systematic technique that follows predefined rules to identify defects
B) An informal technique based on tester experience and intuition to anticipate potential defects
C) A formal method that requires comprehensive documentation of all test cases
D) A technique that guarantees finding all defects in a system

Answer: B
Explanation: Error guessing is fundamentally informal and experience-based. Option A is wrong because error guessing is not systematic with predefined rules. Option C is wrong because it doesn't require formal documentation. Option D is wrong because no testing technique guarantees finding all defects.

Question 2: "When is error guessing most appropriate to use?"

A) As the only testing technique for a safety-critical system
B) To supplement formal testing techniques in identifying high-risk defects
C) When detailed requirements are not available
D) When you have inexperienced testers with limited domain knowledge

Answer: B and C (accepting either depending on question format)
Explanation: Error guessing is most appropriate as a supplement to formal techniques (Option B) and when information is unclear or limited (Option C). Option A is wrong because error guessing shouldn't be the only technique for critical systems. Option D is wrong because error guessing depends on experienced testers with domain knowledge.

Question 3: "What is a key difference between error guessing and equivalence partitioning?"

A) Error guessing is systematic; equivalence partitioning is informal
B) Error guessing is informal and not repeatable; equivalence partitioning is systematic and repeatable
C) Error guessing requires formal documentation; equivalence partitioning does not
D) Error guessing is used only for security testing

Answer: B
Explanation: This directly addresses the core difference. Error guessing is informal and not repeatable because it depends on individual tester intuition. Equivalence partitioning is systematic and repeatable because it follows defined rules. All other options reverse the characteristics or introduce irrelevant information.

Key Takeaways for the Exam

• Error guessing is informal, experience-based, and intuition-driven testing
• It should complement, not replace, formal testing techniques
• It's particularly useful for high-risk areas, unclear requirements, and rapid defect identification
• Results depend heavily on tester experience and domain knowledge
• It's not systematic, not repeatable, and not formally documented
• Common error categories include boundaries, input validation, calculations, error handling, state transitions, integration issues, performance, and security
• When comparing to other techniques, remember the systematic vs. informal distinction
• Focus on practical application: how would experienced testers use it?