Ideal Hours vs. Everything Else
Your work breakdown seems to be based on ideal hours. While there are proponents of this approach, ideal hours rarely track with any precision since estimates are usually a distribution. Another person said:
[P]recision doesn’t equal accuracy and accuracy is what we’re really after.
In other words, whether any given task will take 30 minutes of wall-clock time or not is an educated guess, and doesn't really tell you how much process overhead, task-switching time, or other issues will impact delivery of a given unit of ideal time. The real underlying question is how many ideal hours one can actually expect to complete in any given cycle, and your WBS doesn't really say anything about that.
Granularity vs. Clarity
Granularity should result in clarity on several key points:
- Success criteria.
- Implied or explicit failure criteria.
Your WBS doesn't address any of those things. For example, what are the dependencies of "Export to Excel?" How will you know if it's done? How will you know if it was done right?
These things matter because your time estimates should be based on those things. Perhaps you and your team have some implicit understanding of what that task means, and how it fits in with everything else, but it is unlikely to be documented or transparent enough to make the estimate of "5 hours" anything but an opaque guess based on indeterminate criteria.
Note that this doesn't mean you have to decompose the task further. It may or may not already be granular enough. The real issue is that it isn't clear.
Have the Right People Estimate
Finally, in Scrum the cross-functional team provides the estimates because they are the ones doing the work. As a result, they are the only ones who know what skills and tools they have at their immediate disposal for the task, and whether or not they have sufficient confidence in the estimate to commit to completing the work within a given time-box.
Confidence in estimates grows over time. The right questions to ask are:
- Whether the WBS you presented is sufficient to give you and your team confidence in the estimates.
- The granularity of individual WBS elements is sufficient to track progress and deviations from the schedule.
- The clarity of the WBS elements is sufficient to communicate effectively within the team, and between the team and the organization.
Definitely ask those questions. However, the only people who can answer them already work with you; the answers can't be found on the Internet.