4. Phased strategy

This is an useful strategy for systems that are made up of smaller sub-systems. The idea is to introduce each sub-system one at a time.

For example, module 1 in the picture above is activated in the system. This replaces part of the existing system. The sub-system is thoroughly tested by the users to ensure that there are no serious issues with it.

Then the next phased introduction continues by loading module 2 into the system. The same thing happens again - problems and issues are identified and resolved before carrying on.

The main disadvantage of this strategy is that it may not be possible to sub-divide the system in this way. Another issue is that it may take a long time to roll out every module. A more subtle problem may be that a fault in 'module 2' causes an unexpected fault in module 1 that previously worked fine.

Its attraction is that it offers very controlled management of risk. If a module does not work properly, then the older part of the system can be re-instated without too much disruption to the organisation.