Using top-down estimates is good for initial and strategic decision making or in
situations where the costs associated with developing better estimates have little
benefit. However, in most cases the bottom-up approach to estimating is preferred
and more reliable because it assesses each work package, rather than the whole
project, section, or deliverable of a project. Estimating time and costs for each
work package facilitates development of the project schedule and a time-phased
budget, which are needed to control the project as it is implemented. Using the estimating
guidelines will help eliminate many common mistakes made by those unacquainted
with estimating times and costs for project control. Establishing a time
and cost estimating database fits well with the learning organization philosophy