The most effective way of demonstrating that software does what is intended is to run it and check the results. Doing this "by hand" is, naturally, time consuming and expensive. Leaving this until the last minute means that it takes too long to track done the cause of problems and fix them.
We advocate automating both the running of the software and the checking of results. In addition this should be integrated into the development process to ensure that everyone is sure that the software is working at all times.
Finding the appropriate tools and designing the software to facilitate testing can be challenging, but we know it can be done - because we've done it.
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