These levels of testing are performed when a system is being built from the components that have been coded. There is another level of testing, called regression testing, that is performed when some changes are made to an existing system. We know that changes are fundamental to software, any software must undergo changes. Frequently, a change is made to “upgrade” the software by adding new features and functionally. Clearly, the modified software needs to be tested to make sure that the new features to be added to perform intended work. However, as modifications have been made to an existing system, testing also has to be done to make sure that the modifications has not had any undesired side effects of making some of the earlier services faulty. That is, besides ensuring the desired behavior of the new services, testing has to ensure that the desired behavior of the old services is maintained. This is the task of regression testing.

For regression testing, some test cases that have been executed on the old system are maintained, along with the output produced by the old system. These test cases are executed again on the modified system and its output is compared with the earlier output to make sure that the system is working as before on these test cases. This certainly is a major task when modifications are to be made to existing systems.

Regression testing is therefore, done to ensure that program is performing as per specifications.

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