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Bottom-up estimating requires assessing detailed components before summing for total duration. This approach involves breaking down the project into its smallest work packages or individual tasks. Each task is estimated individually for duration, effort, and resources required, and then these individual estimates are aggregated to form the total project estimate.

This method ensures a high level of accuracy as it takes into account all specific activities and their requirements, allowing for more precise planning and resource allocation. By evaluating each component in detail, it provides a comprehensive understanding of the project scope and can highlight potential risks or resource constraints early in the project planning process.

Focusing on only high-level work packages would not give the detailed insight necessary for accurate estimation. Limiting the approach to only external resource costs would ignore internal labor and other expenses that could be crucial. Additionally, using historical data exclusively could lead to inaccuracies if past projects do not reflect the current project's unique circumstances and variables.