The traditional approach in software development is to determine all the features for the release up front and...
estimate how long it will take to complete the design, development, and test of those features. The tasks required to complete these efforts are mapped out and an end-date is estimated. Once again, both features and schedule are taken into account, but this approach focuses more on the features, and often the initial end-date is a target and the schedule is adjusted if requirements change or there are delays in finishing the development efforts of all the features.
As Robin Goldsmith explains in Reliably estimating the requirements effort, part 1, there are different ways in which we estimate. "Top-down estimates make a single estimate for the whole thing and allocate that estimate among the major work pieces." In this case, a final date might be estimated and then the project manager would work backwards to determine what tasks need to be completed to accommodate that schedule.
In the second part of this series, How to complete requirements assessments from the bottom-up, Goldsmith defines bottom-up estimates as an approach that "breaks down the expected work into task and sub-task pieces, estimates each piece, and rolls up the estimates into the estimate for the total." This approach is more feature-driven.
Both of these approaches need to take into account both the product features and schedules when building a plan. Each plan you make will include tasks that must be completed and an estimated release date. Both of these are important for a successful release. The flexibility you have with each will depend on your project. The stakeholders should weigh-in with their thoughts on the importance of features vs. schedule, and the priority of each.
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