Levels of Testing

Different levels of testing are used in the testing process; each level of testing aims to test
different aspects of the system. The basic levels are unit testing, integration testing, system testing, and acceptance testing as shown in Figure below:

Figure: Levels of Testing

• Unit Testing:
  The first level of testing is called unit testing. The goal of unit testing is verification of the code produced by individual programmers, and is typically done by the programmer of the module. A module is offered for integration only after it has been unit tested satisfactorily.
• Integration Testing:
  The next level of testing is often called integration testing. In this, many unit tested modules are combined into subsystems, which are then tested. The goal here is to see if the modules can be integrated properly. Hence, the emphasis is on testing interfaces between modules. This testing activity can be considered testing the design.
• System Testing:
  The next level is system testing. Here the entire software system is tested. The reference document for this process is the requirements document, and the goal is to see if the software meets its requirements.
• Acceptance Testing:                                                                                                                The next level is acceptance testing. Acceptance testing is often performed with realistic data of the client to demonstrate that the software is working satisfactorily. It may be done in the setting in which the software is to actually function. The goal of acceptance testing is to test if the system satisfactorily solves the problems for which it was commissioned.