Expert estimation of software development effort may follow top-down or bottom-up strategies, i.e., the total effort estimate may be based on properties of the project as a whole and distributed over project activities (top-down) or calculated as the sum of the project activity estimates (bottom-up). The study reported in this paper examines differences between these two strategies based on measurement and video recording of the discussions of seven estimation teams. Each estimation team applied a top-down estimation strategy on one project and a bottom-up estimation strategy on another. The results from the study contribute, we believe, to an improved understanding of when to apply top-down and when to apply bottom-up estimation strategies.