Parallel testing is used whenever you are replacing an older application with something new. In this instance you will do all your processes TWICE. Once in each system. The basic question here is simple. Do you get the same results out of both systems? If not, is that what you expected? Part of the reason for the replacement might be improvement and correction. But in most of the application, you should get the same results or be able to explain why based on new functionality.
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