MS Project: Linking and/or Assigning Resources to Summary Tasks
It is important to understand that summary tasks in Project are not activity tasks but rather a visual and data summary of the subtasks under them. Two often asked questions about summary tasks are, "can resources be assigned at summary level" and, "can summary tasks be linked"? The simple answer is yes, to both questions, but doing so is NOT recommended. Why? Here are some important considerations for each.
Summary Task Resources -
Assigning resources to summary tasks is probably the number one reason why values in the Cost field do not appear to add up. Summary task assignments create double booking if the resource is also assigned to a subtask. For example, a supervisor is assigned at summary level to provide supervision for all subtasks of that summary and the supervisor is also a working team leader for the group so he/she is assigned to one or more tasks under the summary. It will cause data to be hidden, causing confusion with values, because data from subtask fields (e.g. Cost, Work, etc.) is added to resource data at the summary level.
The one case where a resource assigned to a summary task may be appropriate is for management/supervisory support. Just be sure that support is not also assigned to any subtasks under that summary. An alternative option is to create a separate task to delineate management/supervisory support. This can be a single task that spans the entire project, or it can be a recurring (e.g. month to month) series of tasks. When done as a single task spanning the full duration of all other subtasks, a hammock task (reference: http://social.technet.microsoft.com/wiki/contents/articles/32040.ms-project-hammock-tasks.aspx ), might be a viable option. Another option, when the overall project plan exists in a group of multiple independent project plans, is to create a separate file for management tasks.
Summary Task Linking -
Linking summary tasks is probably the number one reason project files develop “circular reference” errors which can be very difficult to find and correct. The main cause is links on both the summary and between subtasks under different summaries. Other causes include outdenting, where a linked subtask becomes a linked summary, and dragging and dropping linked subtasks from one summary grouping to another. Another major problem is that the logic flow of tasks becomes very complex as summary links conflict with subtask links and from the user's standpoint, the schedule doesn't seem to make sense or doesn't respond to changes/updates as expected.
However, the one time links on summary tasks MIGHT make sense is for a very high level plan that is developed for initial planning purposes. That is, summary tasks are linked to show the overall schedule logic of groups of tasks but there are NO links between subtasks under different summary tasks.
References
This is a revision of the Project MVP FAQs 48 & 49: http://project.mvps.org/faqs.htm#Summary%20Task%20Resources