3. Parallel

This is a very popular method as it is relatively low risk. The idea is to run the old system alongside the new system for a while.

With this approach, the outputs or behaviour of each system can be compared to make sure that the new system is performing as expected. If a serious problem is found, then the organisation can just carry on using the existing system until a fix has been put in place.

The disadvantage of the parallel method is that it is effectively duplicating effort, which costs time and money. Also there may be specialist hardware or equipment that both systems need to use and this may be a problem.

For these reasons, the parallel strategy may not be the best choice for a particular project and so one of the other three approaches may have to be used.