Understanding the Four Levels of Software Testing

Software testing is an essential aspect of the software development life cycle (SDLC). Creating a software product is not enough, it is crucial to test it and make sure it meets performance requirements and is free of any defects before it can be released to end customers. Proper test case management and a test management tool like Testworthy can help simplify this process.

The software testing phase involves a combination of manual and automated testing to verify that the software product meets expectations and does not contain any bugs. Good software needs to be secure, reliable, error-free, responsive and easy to use, and software testing ensures that it meets these criteria. Thorough software testing procedures are crucial for creating flawless products and happy customers.

Generally, software testing can be divided into 4 main testing levels. These are:

  • - Unit testing
  • - Integration Testing
  • - System Testing
  • - Acceptance Testing

Each level of software testing is important to check that the software works seamlessly from start to finish. Good test case management practices are key to ensuring that each level of software testing flows smoothly. A test management tool like Testworthy can aid software testing teams in managing their projects better. Let us now explore the four software testing levels in more detail.

Unit Testing

The first level of software testing is unit testing. This is the lowest level and is typically done at the earlier stages of development.

Units refer to small blocks of individual code. In unit testing, each individual coding unit or component is tested separately to ensure that it does not show any defects. Unit testing is a form of white box testing which means that the internal code is checked and verified. Since unit testing is done at a lower scale and is dependent on coding language, it is performed first-hand by programmers who write the code rather than the software testing team.

Performing unit testing manually requires too much effort and time, hence unit testing can particularly benefit from test automation. Automating unit tests increases software testing speed, yields more accurate and reliable results and provides better coverage.

levels of software testing

Integration Testing

Although unit testing confirms that the individual units of code are working independently, this does not mean that they will continue to do so once they come together. As the units combine, they have various interactions and co-dependencies that need to be tested. This is where integration testing comes in. Integration testing ensures that when different software modules are integrated, they still work together flawlessly. Hence integration testing is more related to the link and data flow between modules or pieces of code.

For example, different developers and teams may code different parts of the software. They will each have their own unique logic and way of coding. Once these different modules come together, it is important to ask, are they working in harmony with each other? Are they compatible and functional? Is the software working correctly as per requirements? Integration testing verifies that once everything unites, the software continues to work efficiently.

Some of the well-known approaches to integration testing include:

  1. 1) Big Bang Testing: All modules are integrated and tested together in one go. However, this means that testing can only take place once all modules are developed and completed.

  2. 2) Incremental Testing: Different modules are integrated and tested incrementally i.e., added one by one. Incremental testing can be sub-divided into top-down incremental testing (higher level modules are tested and integrated first), bottom-up incremental testing (lower-level components are tested and integrated first) and sandwich or hybrid testing (a combination of the top-down and bottom-up approaches).

    System Testing

    After integration testing, system testing is the next step. System testing verifies the complete system or product before it goes into the market.

    System testing is a type of black box testing that checks the product as a whole from end to end, testing both functional and non-functional aspects of the software. It tests the entire application to make sure it meets all quality benchmarks, client requirements and specifications, whether technical or non-technical. It ensures system operations, overall performance, and smooth interaction between modules and subsystems.

    System testing is done in a production-like environment to make it as accurate and close to real life conditions as possible. System testing can consist of various types of testing that check different parameters such as performance, security, load, stress, volume, reliability, usability, compatibility etc.

    Acceptance Testing

    The last level of software testing is acceptance testing. What is unique about this type of testing is that it is actually performed by the end-users themselves rather than software testers.

    Acceptance testing is a form of black box testing where customers check the functionality, verify the features and make sure that the final product is working as expected. Hence acceptance testing is performed from the user’s viewpoint to determine whether the software meets the client /user requirements and their acceptance criteria and determine whether the larger market will accept the product or not.

    Acceptance testing may include checking the usability and user interface (i.e., whether relevant information is easy to find, software is intuitively navigable, features facilitate ease of use etc.,), checking functionality (the software has all required the features, works without error and performs functions as expected) and checking technical aspects (the product meets all technical specifications defined by clients).

    Overall, acceptance testing ensures that there are no unexpected errors, delays or confusions in the software, the software meets end-user requirements, complies with laws and regulations, is in accordance with the business contract and meets business objectives.

    Feedback from the acceptance testing stage is used to enhance software performance and fix bugs before the software is released. Since it is hard to roll back changes once a product has gone live, the final software is released only once complete satisfaction is achieved in acceptance testing stage.

    The drawback to acceptance testing is that since customers are not trained software testers, they might not cover all critical scenarios, or test comprehensively. Acceptance testing is dependent on users and the software might have to undergo several changes before the end product is finally accepted and can be released.

    Going Through the Levels

    Software testing levels can further be understood through an example.

    Consider the process of a laptop assembly. The various units in the laptop, such as monitor, webcam, CPU, keyboard, screen, graphic card, etc. will be created and tested separately; this is unit testing. Then these components are brought together and connected one by one in groups to and see if they work together. For example, the webcam is connected to the laptop to see if the video displays on the screen or not. This is integration testing. Then, once all components are integrated and the laptop is ready, it is tested to see how it works overall, i.e., system testing. Finally, the laptop is handed over to the end-users who use it and share their experience and feedback, and this is acceptance testing.

    Hence the 4 software testing levels i.e., unit testing, integration testing, system testing and acceptance testing, are performed in sequence. Starting from the most basic and simple testing level to an overarching and all-encompassing testing of the entire software, going through the 4 software testing levels ensures the creation of high-quality products.

    Apart from the four main testing types, regression testing also plays an important role in the testing lifecycle. After a software is released, it undergoes many updates and changes. Regression testing ensures that any new changes are not negatively affecting the existing software and its operations. However, regression testing is not a separate software testing level but rather, it can be performed at any of these 4 stages as it is more related to software maintenance.


    The four levels of software testing check the software from start to end, validating it and making sure that it works smoothly before it is finally released. Going through the 4 software testing levels can be made easier with the help of test management tools, and proper planning and test case management. By going through these four levels of software testing, QA teams can ensure the production of high-end software that guarantees success.


    The four levels of software testing are unit testing, integration testing, system testing, and acceptance testing. They are performed in the same sequence in order to ensure a holistic testing of the software product.
    The four levels of software testing differ in terms of what they test. Unit testing tests individual software modules or units, integration testing tests how well the modules are integrated once they are grouped together, system testing is an end-to-end testing of the entire product once it is developed, and acceptance testing is the testing done by end-users to determine their overall experience with the product.
    Unit testing is usually conducted by developers who write the code since it involves checking the coding lines and logic. Integration and system testing are performed by experienced software testing teams and professionals. Finally, acceptance testing is performed by real-life users, including end customers or business clients, to get an idea of how well the product will perform in the market.