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This is an important concept designed as a last test to ensure that the system works. It is only undertaken once all testing, training, and data set-up are complete. It consists of:

• ensuring that all files in the new system exactly reflect all files in the existing system;
• running the same data through both systems for a length of time, which is usually no less than a month and commonly is significantly longer. As an example, for accounting systems, it is common to run through two, or three, end of period runs;
• comparing the state of files, printed reports, etc., throughout the parallel-running period and, at the end, identifying any differences and explaining them or, if necessary, correcting them. In the latter case, it may be necessary to rerun some, if not all, of the runs.