According to the PMBOK® Guide, specifically within the Estimate Activity Durations and Estimate Costs processes, Bottom-up Estimating is a method of estimating project duration or cost by aggregating the estimates of the lower-level components of the Work Breakdown Structure (WBS).

When to Use: This technique is utilized when an activity cannot be estimated with a reasonable degree of confidence. In such cases, the work within the activity is decomposed into even more detail.

The Process:

The activity is broken down into smaller, more granular pieces of work.

Detailed estimates are created for each of these lower-level components.

These individual estimates are then " rolled up " or aggregated into a total quantity for each of the activity ' s resources or costs.

Accuracy and Cost: Bottom-up estimating is typically the most accurate estimation technique because it looks at the work at a very granular level. However, it is also the most time-consuming and costly method to perform. The accuracy is often driven by the size and complexity of the activity; smaller pieces of work generally lead to higher confidence in the estimate.

Comparison with other options:

B. Parametric: This uses a statistical relationship between historical data and other variables (e.g., square footage in construction) to calculate an estimate. It is based on unit rates rather than decomposition of work.

C. Analogous: This is a " top-down " approach that uses the values of a previous, similar project as the basis for estimating. it is used when there is limited information, making it the opposite of the detailed decomposition required for bottom-up.

D. Three-point: This technique uses three estimates (Most Likely, Optimistic, and Pessimistic) to account for uncertainty and risk. While it addresses a lack of confidence, it does not involve the decomposition of work into more detail to arrive at the figure.