In my experience it's a horses for courses type of a situation . In case of some projects the Big Bang approach works brilliantly while in some other projects I would settle for the Phased Implementation approach .
A number of factors have to be considered before deciding which approach serves best .
A few of these factors are :
Does the implementation cover a single site or multiple sites?
Does the implementation cover a single business or multiple businesses?
If a phased approach is adopted, what will this mean for integration between the new system and legacy systems during the interim period?
Are there any other competing business activities that need to be taken into account?
What level of risk is acceptable?
Which costs more – big bang or phased?
Some of the Pros & Cons of these two approaches are :
Pros of BIG BANG approach
No one has to operate their business in two different computer systems
· Everyone in the company moves forward on the same day.
· Cuts out the ability for people to say, “oh we’ll fix that issue until we convert fully”
· Shorter implementation time
· Pain and frustrations are condensed into one time period, not drawn out
· Lower costs
· Training needed only on the new system, not a changeover
Cons of BIG BANG approach
· A number of things can go wrong
· More pronounced problems
· Small details or issues can be overlooked in the rush
· Sink or swim for employees when it comes to learning the new system
· Testing can be tough prior to the implementation
· Failures in one part of the system can cause problems and failures in others
· Fall back plans do not always work out
· Performance can temporarily decline after the initial implementation as a result of employees adjusting to the new system
Pros of PHASED IMPLEMENTATION approach
· Employees learn as they go, there is no dip in performance after
· More time for users to adapt to the new system
· Not a sink or swim environment
· Small details and issues can be fixed as you go
· Skills and experience are gained with each step/phase which can help smooth the process as you get further along
Cons of PHASED IMPLEMENTATION approach
Takes longer to be fully converted
· Not as focused as the big bang
· A state of continuous change can sometimes be disruptive
· Can have missing information because each module relies on info from others, so in a transitional period there may be some gaps
· Temporary bridges need to be made from old to new
Depending on the project one may even choose the Parallel Adoption approach . The Pros & Cons of this approach are :
· Least risky
· Users learn the new system while working on the old
· Slower pace than the big bang but faster than a phased roll-out
· Most expensive
· Employees have to enter data in both systems- this can be inefficient and breed data-entry problems
You may also choose to use a Combination of the above mentioned 3 approaches .