The 4 Levels of Testing in Software Engineering Explained

Every engineer needs to know the four levels of software testing and why going through each level is so crucial for a project’s success.

Before your team can release a new application or project, how many rounds of testing has it gone through to check for errors and bugs?

Hopefully, the answer is greater than “zero” tests. Ensuring your software engineers follow a thorough testing process can be the difference between a project that’s a success and one that’s a massive failure.

Before an engineering team gets started working on anything new within the project pipeline, discover the ins and outs of software testing, why this testing is absolutely crucial, and the different levels to take throughout the process.

What is software testing?

Software testing is the process of checking whether the actual software product has all of the expected requirements and ensuring it’s free of bugs, glitches, and defects. This can be done using manual or automated tools, but will always analyze the end result with the requirements given to ensure the final result has everything it needs to have.

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Why is software testing important?

Software testing is an incredibly crucial step for an engineering team to complete. No matter what software or product your team creates, bugs, errors, and glitches will likely occur. But those errors become less of a headache when they’re identified and solved early—that is, well before the software product delivery. When an engineering team takes the time to test software, it ensures reliability, high performance, and security, saving time and money and ensuring the customer is happy, too.

The different types of software testing

Typically, software testing can be broken up into three categories:

  • Functional testing: A type of software testing that validates the software against a list of requirements or specifications. These tests aim to assess each application function and mainly involve black box testing. Functional testing analyzes details like user interface, database, security, application programming interfaces (APIs), and client/server communication.
  • Non-functional testing: Sometimes called performance testing, this method focuses on testing an application for its non-functional elements and requirements. Non-functional elements encompass things like performance, reliability, and usability. An example of a test would be checking how many users can be logged into a tool simultaneously without the tool lagging or glitching.
  • Maintenance testing: A method of software testing that aims to uncover software errors or bugs within a program after changes have been made. It retests the program to ensure that after the changes are complete, the changes don’t negatively impact the features within the code or conflict with necessary requirements.

What are the different levels of software testing?

When it comes to the different levels of software testing, there are four that take place. Each level checks for something different, and each of them must be conducted thoroughly—nothing should be rushed or skipped.

1Unit testing

Unit testing is a software testing process for testing specific units, components, or software elements. This is the most basic type of testing, and the goal for this level of testing is to validate that each unit of code performs how it should and is free of bugs, errors, and glitches. 

Typically, this level is done during the development (or coding) phase by software developers who isolate a section of code and verify that it’s correct. It’s considered a WhiteBox testing method, meaning the code can be viewed but usually not altered, and should always take place early in development to save money, time, and effort in the long run.

With unit testing, the earlier, the better! In many cases, this testing is completed by developers before they hand the code over for further testing.

2Integration testing

Next is integration testing, which is when different software components and modules are combined and tested as a group to make sure everything is ready for the next level. Since a standard software project will likely consist of various modules, coded by multiple programmers, the goal is to test to expose potential defects or bugs between the various modules. Sometimes, this phase is referred to as I & T (integration and testing), thread testing, or string testing.

This method focuses on checking the data flow from one module to the next and is performed by testers. It’s another crucial level of testing because it verifies everything is working as one singular unit.

3System testing

The next level of software testing is system testing. This checks for a system’s compliance in accordance with the necessary given requirements. System testing inspects components like performance, load, reliability, and security with the goal of evaluating the end-to-end system specifications. 

Typically, this method is done by a professional testing agent on the completed software product before it can be introduced to the market with real users. This step is important because the project is so close to being complete, so it should be tested in an environment similar to what the user will experience once it’s finished.

4Acceptance testing

Finally, there’s acceptance testing. What makes this level different from the rest is that it’s conducted by the user or customer since the goal is to find out if the requirements have been met on delivery of the final product. Since acceptance testing is the final phase, it needs to validate the end-to-end business flow and check for things like cosmetic errors, spelling mistakes, and usability.

The testing sequence 

These four types of testing need to be done with care and should be methodical and deliberate. These tests need to be completed in order as this sequence will help to reduce the number of bugs or errors that pop up before the final product is launched.

Every software engineering team knows how important testing can be throughout every phase of the development cycle, and skipping or rushing a level can cause a project to be past due and over budget.

By going through each level, starting with the simpler components, and moving towards more complex elements, testers and engineers can feel confident that the software is being thoroughly and properly examined in an efficient manner. Hopefully, doing so will also result in a more satisfied customer or client.

Testing principles for software engineers 

There are some testing principles that software engineers can keep in mind. Some of these principles include:

  • Always keep the requirements of the customer in mind and make sure they’re traceable and identifiable during all levels of testing.
  • Before you start testing, create an outline of how the tests will be conducted.
  • Consider applying the Pareto principle to software testing, meaning 80% of all errors that are identified during testing will likely be traced to 20% of all program modules.
  • When you start testing, begin “in the small” and continue until you progress into testing “in the large.”
  • Remember that it’s simply impossible to test all data combinations–your energy should be spent elsewhere.
  • Involve an independent third party if necessary, since testing should be as effective as possible.

Testing, testing, 1… 2… 3…

Test early and test often.

Let that be the motto of your software engineering team. A systematic approach to testing is bound to happen when you adopt this level of scrutiny and attention to all details within projects. Being able to pinpoint faults and errors within a project sooner will always mean less time and money wasted by not only your team, but also the company as a whole.

Pinpointing errors early will always take less effort than fixing a bug the day before launch. So roll up your sleeves and make testing a priority.


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About the author

Mara Calvello

Mara Calvello is a freelance writer for Fellow, in addition to being a Content Marketing Manager at G2. In her spare time, she’s either at the gym, reading a book from her overcrowded bookshelf, enjoying the great outdoors with her rescue dog Zeke, or right in the middle of a Netflix binge. Obsessions include the Chicago Cubs, Harry Potter, and all of the Italian food imaginable.

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