Top down estimating or also known as Analogous Estimating looks at either a project or system from the concept stage and uses information from similar projects or items to give an overall cost. This type of estimating will only give a ball park estimate with large variances in cost.
Bottom down estimating is more accurate as each task is broken down into smaller individual tasks which can then be estimated. These smaller estimates can then be collected together to give an accurate overall estimate.
Bottom down estimating is more accurate as each task is broken down into smaller individual tasks which can then be estimated. These smaller estimates can then be collected together to give an accurate overall estimate.