Which estimating method requires estimates to be made at the detailed work activity level of the WBS?

The estimating method that requires estimates to be made at the detailed work activity level of the Work Breakdown Structure (WBS) is bottom-up estimation. This approach involves breaking down a project into its smallest components or activities and estimating the costs, resources, and duration required for each of these individual tasks. By gathering detailed estimates at this granular level, project managers can achieve a more accurate overall project cost and timeline.

In contrast, top-down estimation methods involve making estimates based on an overall view of the project, often deriving figures from previous similar projects or expert judgment without focusing on the finer details of each activity. Parametric estimating uses statistical relationships to calculate estimates based on variables and historical data rather than detailed activity analysis. The apportion method allocates costs based on overall project estimates to different components rather than starting at the activity level.

Therefore, bottom-up estimation is the most suitable choice when detailed work activity-level data is essential for creating accurate project estimates.