The Define Activities process is used to further work on the work packages of the work breakdown structure. You use this process to break down a work package further to the activity level. An activity of a project is a small enough unit that you can estimate, schedule, monitor, and manage efficiently and effectively.
Often, the define activities process is merged with the create work breakdown structure process by drilling down the work breakdown structure itself to the activity level instead of stopping at the work package level. The next important process that follows the define activities process is the Create the Network process. One of the reasons that you might not want to take the project to the activity level is because of the fact that creating a network diagram based of the large number of activities defined in this process results in a large network diagram that you might find difficult to manage. As a result, you might want to decide on creating a network diagram based on the work packages identified in the work breakdown structure.
Whether you create a network diagram from the work packages identified in the work breakdown structure or form the activities you have defined after drilling down the work packages of the work breakdown structure, none of these approaches is incorrect. However, it is always advisable to define activities before you derive a network diagram and manage the project more effectively.
To define activities for the project, all you need is an approved scope statement and the work breakdown structure for the project as an input for the process. The approved work breakdown structure includes the work breakdown structure dictionary as well as the work breakdown structure diagram. It is always advisable to involve your team when defining activities for the project to make sure that you define complete and accurate activities. This would make sure that your estimates are more accurate and realistic.
There is no hard and fast rule when you should plan the lower level details. You can either plan it in advance or plan for the higher level and then wait till you start with the project and things become clearer before you plan for the lower level. The latter approach is however more recommended because the former one might result in rework and waste of efforts. The process of planning at the later stages as the project proceeds is known as rolling wave planning because it involves multiple waves of planning as and when various details are finalized.
Whatever option you select for the project planning, you must be confident about it. Later down the line, you must not put it forward as a reason for improper planning and failure of the planning. After you have complete the define activities process, its outcome is the list of activities and their details in the form of activity attributes. Defining activities would helpful in defining milestones of the project in due course of the project life cycle, which are significant for the overall project schedule.