I have never had much experience with this, however, in my opinion, I feel as though bottom up is superior to top down since this has more input from the project team and they are the ones who most likely have the most knowledge and experience on what they have to do and therefore may produce more accurate estimates as opposed to the top down approach. Also more input from the team will make them feel like you value them and their opinion which will get more buy in from the team and will make them more invested in the success of the project. Hope this helps.

I believe that a bottom up approach is the most useful in allowing each project task to be completed in the correct amount of time while not running into expected delays. In an instance where the project completion date is not incredibly stressed, this option may work best because it inhibits rushing the smaller stages as it passes through the proper safety checkpoints. I can only see a top down approach working when there is a hard project deadline that needs to be met. For example, if you need a new product launch by the end of quarter, you would then need to budget your resources according to those constraints because those are unmoving, fixed requirements.

Bottom-up or Top-Down estimation.

According to the PMBOK guide, the WBS or Work Breakdown Structure can be exemplified by two primary approaches, which are top-down estimating, which relies on organization-specific guidelines. In contrast, the bottom-up estimating approach can be used to group subcomponents (Project Management Institute, 2017).

The top-down approach creates a more accurate and complete WBS, while the bottom-up approach is similar to a brainstorming session where team members identify tasks needed to complete the project. I agree that the top-down approach is the most preferable of the two systems in that it works to identify, define, and dissect a solution and then breaks down the tasks necessary to complete the project (Projectmanagement.com, 2022).

Which WBS do you prefer and why?

References

Project Management Institute. (2017). A guide to the project management body of knowledge (pmbok® guide)–sixth edition (Sixth ed.).

Projectmanagement.com. (2022, February 22). Work breakdown structure (wbs): Top-down or bottom-up? Project-Management.com. https://project-management.com/work-breakdown-structure-wbs-top-down-or-bottom-up/

I don't have any experience with these estimations but I believe the bottom-up estimations seem preferred because it is more reliable. You have all the tasks well defined and detailed estimates. It is easier to present an appropriate deadline for the project with this information already provided. It is useful to facilitate the development of the project and keep in mind its budget and schedule before the project beings.