Top-down and Bottom-up Estimation Methodologies

This video from Texavi's Pardha Mantravadi dwells on the top down and bottom up estimation techniques. Top down estimation involves estimating by analogy, i.e., relatively quick and easy to do. Its ideal for early lifecycle phases (Pre-project, Feasibility and Requirements). Uses visible/perceivable features to estimate size. An example of the Top-down estimation is senior management’s view on cost estimates of new project (based on earlier, similar project)

Bottom-up estimation on the other hand is when each component is broken into individual elements and effort is then estimated against each component and accumulated. Decomposition of components could be by tasks (work breakdown structure - WBS) or by modules (product / architecture breakdown).

For specific details and differences, including the examples, watch the video.