The project managers confident in their experience with similar projects are often using top-down estimating based on analogous projects. The problems arise if the results need to be adjusted to a new project. The bottom-up estimating leads to greater accuracy.
Bottom-up estimating is the calculation of project duration and cost from de-tailed data about the activity and its cost and time requirements. Accuracy is achieved because the estimates for each component of the activity are given by the individuals responsible for these components.
The project activities are broken down into smaller components and an estimate of duration and cost is assigned to each component. The schedule is determined by aggregating the individual duration estimates, while the budget is determined by aggregating the individual cost estimates.