How does one determine the number of FACT tables that are required in a data-warehouse?

When analysing data sources that need to be included in the data-warehouse (Kimball), how does a developer determine whether a schema should be a star type or constellation type? Business processes which can be reflected at one of 3 levels (transaction, periodic snapshot or accumulating snapshot). However, there can exist business processes where the captured data is effectively identical, at any of these levels for any number of these processes. What are the remaining considerations? (Performance..)