4 Levels Of Testing In Software Testing: Explanation & Examples

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Relia Software

Quoc Bao

Relia Software

Software Development

4 levels of testing in software testing are: unit for single code, integration for modules, system for overall functions, and acceptance for user feedback.

levels of testing in software testing

Table of Contents

Software testing plays a critical role in ensuring software quality. By detailedly reviewing specifications, design, and code, testers can find and remove mistakes to maintain and upgrade the quality of the software during the testing cycle. Different phases or stages of testing during the software development cycle are called testing levels. 

This blog will explore 4 levels of testing in the software testing life cycle with their benefits, challenges, and examples.

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Level 1: Unit Testing

Unit testing is a fundamental software development strategy where individual code units (functions, methods, classes) are isolated and tested independently. This testing level focuses on verifying each unit's internal logic and functionality to ensure they operate as expected.

Who Will Perform The Test?

  • Developers: Develop the test to build cleaner, more manageable code, detect errors early, and refactor confidently.

  • Technical architects: Help create tests and encourage best practices during code reviews. 


  • Improving Code Quality: Unit testing promotes well-structured and maintainable code.

  • Early Detecting Bug: It can identify errors early in the development cycle, making them simpler and less costly.

  • Refactoring Confidence: Unit tests will detect unexpected regressions to help developers rewrite code more confidently.


  • Resembling Dependencies: Unit tests frequently need to mimic external dependencies, which can be difficult to set up.

  • Testing Coverage: It might be difficult to get complete unit test coverage, especially for complex codebases.

  • Testing Maintenance: Unit tests must be maintained and updated with code changes to ensure their effectiveness.


Imagine testing an edge case in calculations function:

def add(x, y):
  """Adds two numbers together and returns the sum."""
  return x + y

# Unit test for the add function
def test_add():
  assert add(2, 3) == 5  # Test case 1: positive numbers
  assert add(-1, 1) == 0  # Test case 2: negative and positive numbers
  assert add(0, 0) == 0  # Test case 3: zero values

Level 2: Integration Testing

Integration testing verifies how individual software components (modules) work together after being unit-tested. It focuses on ensuring individual software pieces. In this level of testing, two or more unit-tested modules are integrated to test technique-interacting components. Then, the results will show whether these modules perform as expected or not.

Who Will Perform The Test?

  • Developers: Identify integration points between components; Collaborate on test cases to validate module interactions; Fix problems discovered during integration testing.

  • Testers (QA): Organize, conduct, and review integration tests; Report and track integration test results.

  • Technical architects: Plan test strategy with modules prioritized depending on risk and criticality; Design and manage test environments that accurately represent integration scenarios.


  • Early Integration Issues Detection: Detect flaws that would not be found during unit testing, such as incompatible interfaces, data exchange difficulties, or timing issues across modules.

  • Stability Improvement: Ensure that modules work cohesively, leading to a stable software system.

  • Reduced Regression Risk: Ensure a module change does not affect other integrated modules. 

  • System Functionality Confidence: Increase confidence that the entire system functions properly.


  • Complexity Increase: Testing interactions across several modules can be more difficult than unit testing, necessitating careful preparation and cooperation.

  • Dependency Management: Integration testing may be hampered by reliance on external systems.

  • Test Case Design: Developers must understand module relationships and potential integration points to create successful test cases for integration testing.

  • Time Consumption: Setting up test environments and managing complex scenarios can be time-consuming.  

Best Practices

Identify Integration Points: Identify the interfaces and data exchange points between tested components.

Start Incremental Integration: Start by integrating a few modules and gradually increase the complexity.

Mock External Dependencies: Utilize mocking techniques to replicate their behavior during testing if external dependencies are not available.

Automate Test Execution: Automate integration tests whenever possible to shorten the testing process and assure consistency.

Focus Key Functionality: Prioritize testing core functionality and module interactions first.

Clear Documentation: Record the integration testing approach, test cases, and expected outcomes for future reference and maintenance.


Imagine testing an e-commerce application in which a user adds things to their shopping basket. Integration testing would include:

  • Testing the interaction between the product catalog module and the shopping cart module to confirm that accurate product information and prices are entered.

  • Ensuring that the shopping cart module communicates successfully with the user interface module to display the added products and their quantities.

  • Simulating interaction with a payment gateway to confirm that the shopping cart can initiate a safe payment transaction.

integration testing
Integration Testing in Software Testing (Source: Internet)

Level 3: System Testing

System testing is a comprehensive evaluation of a software system's overall functionality and performance. It ensures that the entire system meets the development criteria. This level of testing is critical for detecting system-wide issues such as compatibility problems, performance bottlenecks, and security weaknesses.

Who Will Perform The Test?

  • QA Testers: Create and run detailed test cases, analyze findings, find faults, and report them to developers or specialists.

  • Technical Architects: Help organize test phases, prioritize modules for integration testing by risk, and build test environments that simulate real-world integration scenarios.

  • Domain Experts: Verify that software in specialized fields (banking, healthcare, etc.) meets industry standards and regulations.

  • End-Users: Provide input on the system's usability, functionality, and user experience issues in the final stages of testing.

>> Read more: Web Design Tips To Elevating Your Website's User Experience


  • System Quality Improvement: Help detect and address major issues that may influence the overall quality and functionality of the product.

  • UX Issues Recognization: Identify usability issues and ensure that the system is intuitive and easy to use.

  • Reliability Strengths: Identify performance bottlenecks, memory leaks, and other issues that may affect the system's stability and reliability.


  • Time-consuming: Be costly and time-consuming due to the complicated test settings and scenarios.

  • Test Environment Complexity: Be difficult to create a realistic test environment that closely resembles the production environment.

  • Requirement Coverage: Be challenging to ensure that all system needs are thoroughly tested.

  • Limited Scope: Focus on overall system functionality rather than specific components.

Best Practices

Clear Test Objectives: Define system testing goals and scope based on system requirements and user demands.

System Design Documents: Use system design documents to identify critical components, functionality, and testing risks.

Set Test Case Priority: Prioritize testing core functions and user workflows.

Multiple Testing Methods: Combine testing methods like black-box testing, performance testing, and security testing to ensure comprehensive coverage.

Test with End-Users: Gain valuable feedback on user experience and real-world functionality feedback with user acceptability testing (UAT).

Record Test Results: Record test findings, issues, and solutions for future reference.


Consider testing a social networking app in the scenario of uploading and sharing a post.

Test Objective: Ensure that the complete process of generating, uploading, and publishing a post with text, photographs, and video material works as expected.

Test Procedures:

  • Step 1: The user logs into the application and creates a post with text content.

  • Step 2: The user uploads an image and/or video attachment. Then, specifies privacy settings and tags relevant users.

  • Step 3: The user successfully shares the post on their feed and relevant profiles. Ensure other users can see and interact with the shared post.

Potential Issues Identified:

  • Image uploading fails due to size or format limits.

  • Video processing issues hinder successful upload.

  • Incorrect privacy settings are applied to the post.

  • Functional issues with user interactions on the post (likes, comments).

system testing
System Testing in Software Testing (Source: Internet)

Level 4: Acceptance Testing

Acceptance testing is the final stage of software testing, which ensures the software functions properly in the user's working environment. It determines whether the software fits the business requirements and user acceptance criteria as expected or not. 

Unlike earlier testing levels, acceptance testing assesses the system from the user's perspective, not the technical functionality.

Who Will Perform The Test?

  • Testers (QA): Be responsible for the planning, execution, and evaluation of acceptance tests.

  • End-users: Participate in User Acceptance Testing (UAT), a sort of acceptance testing that provides real-world input on the system's usability, functionality, and intuitiveness.

  • Stakeholders (clients, product owners, and business analysts): Involved in creating acceptance criteria,  evaluating test findings, and ultimately approving the software for release.

  • Developers: Collaborate with testers to understand the nature of identified issues better, provide technical insights, and ensure reported problems are fixed effectively.


  • Enhanced User Experience: Acceptance testing identifies and addresses usability issues, ensuring an intuitive and satisfying system.

  • Reduced Release Defects: By identifying major issues before deployment, acceptance testing reduces the risk of releasing flawed software that may negatively impact user experience.

  • Increased Trust: Successful acceptance testing fosters trust among stakeholders that the software is ready for production and meets their established requirements.

  • Business Needs Gaps Realization: Acceptance testing can show disparities between designed software and actual business requirements, prompting crucial changes before deployment.


  • Subjectivity of User Acceptance Criteria: User acceptance criteria can be ambiguous and subjective, which makes it difficult for developers to improve the software.

  • Limited Expertise: Testers with insufficient domain knowledge may miss critical user experience issues or struggle to develop effective test cases.

  • Scope Creep: Acceptance testing might result in scope creep and delays if new requirements or changes are not properly managed during testing.

Best Practices

Clear Acceptance Criteria: Define clear, measurable, and achievable acceptance criteria for each system capability based on user demands and business goals.

Early Stakeholder Involvement: Start acceptability testing with stakeholders to gather input and ensure expectations are met.

Prioritize Testing: Prioritize testing important functions and user workflows to maximize efficiency and focus on areas with the highest impact.

Change Management Process: Establish a clear method for handling acceptance testing requirements and changes to prevent scope creep and delays.


Assume a new e-commerce website is being established, and user acceptance testing will focus on the checkout process.

User Acceptance Criteria: 

  • Add items to their carts, review, and modify cart contents (quantity, delete items).

  • Proceed to checkout using various secure and encrypted payment methods.

  • Receive order confirmation emails after completing their checkout.

Testing Approach:

A team of testers will simulate the user experience by:

  • Adding different products to the cart, changing the quantities, and removing items.

  • Going through the checkout process using several payment options while verifying functions such as address entry, payment processing, and error handling for invalid data.

  • Confirming that order confirmation emails are sent with proper order information.

Potential Issues Identified:

  • Discrepancies between displayed and real checkout pricing.

  • Difficulty navigating the checkout process owing to unclear instructions or UI design.

  • Errors occurred during payment processing with specified card types.

  • Order confirmation emails need more critical information or contain inaccuracies.


  • UI/UX design improvement to make the checkout process clearer and simpler.

  • Payment gateway integration is evaluated and updated to correct address card processing issues.

  • Order confirmation email templates are reviewed and adjusted to ensure information accuracy and completeness.

acceptance testing
Acceptance Testing in Software Testing (Source: Internet)

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The 4 levels of testing in software testing – unit testing, integration testing, system testing, and acceptance testing – ensure the quality throughout the software development. By implementing these rigorous testing phases in all of your projects, the tester can uncover any flaws in the system as soon as possible, saving time and money in the long run and, finally ensuring the delivery of high-quality and robust software.

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