Use Two Criteria to Determine Your WBS Estimating Threshold

When you create a schedule you generally don’t know enough to enter all of the detailed activities the first time though. Instead, you identify large chunks of work first, and then break the larger chunks into smaller pieces. These smaller pieces are, in turn, broken down into still smaller and more discrete activities. This technique is referred to as creating a Work Breakdown Structure (WBS).

How small should the activities be before they do not need to be broken down further? This is referred to as your “estimating threshold”. For example, if your estimating threshold was 80 hours, you would continue to break the WBS into smaller entities until all work was less than 80 hours. No work would be left at a higher level.

There are two criteria for determining the threshold.

• Better understanding the work. If you leave schedule activities at too high a level it may not be clear what is required to complete the work. You need to make sure the work is granular enough that it is understandable and it is clear what is required to complete it. For example, if you assign someone an activity that is 240 hours, there may be a lot of work to do for completion, and it may be confusing. If you assign four activities of 60 hours each (or 6 activities of 40 hours each) it should be more clear what is expected for each piece of work.
• Better able to manage the work. When you assign work to a team member you don’t know for sure how he is progressing until the due date (or the completion date if it comes first). For instance, if you assign a team member a piece of work that is due in eight weeks, you are not going to know for sure whether the work is on time until the eight-week deadline. Until that time you can just approximate if it appears things are on schedule. However, eight weeks (or longer) is too long to wait to know for sure if the work is on track. A better approach is to break the eight-week activity into four two-week activities. Then you will know after two weeks if the work is progressing on time or not.

Activities that are to be worked on in the distant future may not be able to be broken down less than the threshold because there may be too much that is unknown about the work itself. The future work can be left at a level higher than the threshold. However, if you leave future work at a high-level, it is still critical to break the work into smaller pieces at least two to three months before you need to start executing the work. This is referred to as "rolling wave" planning. 

These two factors – understanding the work and your ability to manage the work effectively - should drive your decision on how small to make your activities.