The most rapid
Each phase can be fully evaluated before moving onto the next one.
Easy to train staff by letting them learn new skills on the parallel system.
Easy to train staff by letting them learn new skills on each phase as it is introduced.
Less risk of confusion between old and new systems.
Low risk, if a small-scale pilot fails then not too much has been lost.
If there are initial problems with the new system then the old one can still be used.
To look over what you will do in the future.
To look over, study, or examine again.
To study but never examine.
The evaluation stage
The implementations stage
At all stages