Bottom-Up Estimating
An estimating approach that calculates total project cost or duration by first estimating each detailed WBS work package and then summing those estimates to the higher levels.
Key Points
- Starts at the lowest-level WBS components and rolls estimates up to summary levels.
- Typically more accurate than top-down methods, but requires more time and effort.
- Relies on a complete WBS and input from subject matter experts for each work package.
- Creates traceable, documented estimates and a defensible basis of estimate.
Example
A project manager for an ERP rollout asks functional leads to estimate hours and costs for each WBS work package: data migration, interface development, testing, training, and deployment. After gathering these detailed estimates, the PM aggregates them to produce the overall project duration and budget.
PMP Example Question
Your team has finalized the WBS. You ask each work package owner to estimate effort and cost for their activities and then sum these values to determine the project totals. Which technique are you using?
- Bottom-up estimating
- Analogous estimating
- Parametric estimating
- Three-point estimating
Correct Answer: A — estimating by rolling up work package estimates
Explanation: Bottom-up estimating aggregates detailed WBS work package estimates. Analogous is top-down using historical data, parametric uses unit rates, and three-point applies uncertainty ranges to individual estimates.
