The answer to this question is case specific. It will be difficult to generalize the theory for it. For big organizations Big Bang theory is the best choice. Mostly decision on this depends on the readiness audit result, available strength of consulting human power – both internal and external – relevance of ERP at various units, time-frame, and the budget.
The drawback of the big bang implementation approach is that the project is often rushed. On this process sometimes details are overlooked, and sudden changes to business processes may affect negatively. Many times the projects that implement an overly aggressive big bang approach are more risky and result in less satisfaction with the system’s abilities to meet important business requirements.
Therefore it is advisable to go for the key modules such as sales order processing, material planning including purchasing and factory management at the first stage. Modules such as human resources, document management, maintenance and even financials can be implemented later. If there is more than one business unit involved it is best to implement in a relatively easy but not untypical unit first then roll out the implementation to the other units.
