A work breakdown structure (WBS) is a key project deliverable that breaks a team’s work down into manageable chunks. A WBS is a highly visual plan that is presented as a hierarchy. This way, relevant team members can see how the work that is yet to be completed breaks down from the high level all the way to the granular.

Work breakdown structures are only effective if they can accurately and clearly convey the ways in which each deliverable interacts with others for the delivery of the final project. The best structures follow a set of best-practice design principles.

The 100% Rule

The 100% rule for the design of effective work breakdown structures states that the WBS includes 100% of the work defined by the project scope and captures all deliverables in terms of work to be completed.

This means that not only are the final deliverables outlined and expressed in hierarchical terms, but that both external and internal deliverables are explored. This rule applies to all hierarchical levels from the top-level deliverables all the way down to the granular level of work to be completed.

This may seem like an obvious necessity—if activities, tasks, or any work to be completed is left off of a WBS then the document is rendered less effective—but it is an important check to ensure that each level has been explored thoroughly. Additionally, what if the total work to be completed is more than 100 percent? This check helps prevent scope creep and keeps project managers and decision makers on track.

Parents and Children

Top-level tasks or deliverables are commonly referred to as “parents,” while the tasks and deliverables that make them up are referred to as “children.” A good way to ensure that the 100% rule is adhered to is to check each parent deliverable to make sure that the amount of work to be completed by its children equals 100 percent of the parent’s work.

Too little work to be completed is a sign that something has been overlooked. Too much work means the parent task/deliverable needs to be explored further, changed, or broken down into smaller chunks.

Mutually Exclusive Elements

While it is extremely important that the WBS represents 100% of the work to be completed, you must also ensure that there is no overlap of scope, in order for the project to progress smoothly and efficiently and produce quality results.

When various WBS elements overlap in scope, the ambiguity this creates can result in duplication of work, unclear responsibility, and uncertain authority within the project. Additionally, it can create issues in cost accounting and progress reporting—if two elements are not mutually exclusive in scope, completing them both is not a true measure of progress, since work has been duplicated.

To reduce confusion, a best practice is to use hierarchy codes, such as a decimal system with numbers that correspond to deliverables and subtasks, and to use clear and concise deliverable names.

For example, the first high-level deliverable is assigned the number 1. Its first child task would be 1.1, and the child task of that would be 1.1.1. The second child task would be 1.1.2, and a child task of the second high-level deliverable would 2.1, 2.1.1, etc.

Another best practice is to create a WBS dictionary which acts as a key or guide for various work breakdown structure elements such as milestones, deliverables, and scope.

Plan Outcomes, Not Actions

A classic mistake when designing a work breakdown structure is to capture action-oriented details. This will often result in too many, too few, or incorrect actions being assigned to project elements and it can clutter a document that requires crystal-clear clarity. Additionally, too many or too few actions will violate the 100% rule of WBS design, making it an ineffective and flawed document that will only create issues down the road.

Instead, planning outcomes increases the ability of the WBS designer (or design team) to follow the 100% rule and to maintain focus, clarity, and effectiveness.

Know When to Stop Subdividing

Nearly all top-level deliverables within any project will realistically be composed of numerous child deliverables and activities which themselves can be subdivided. It is the responsibility of the WBS designer to know when to stop subdividing and how to present the work to be completed with the appropriate level of detail.

One way of identifying when to stop dividing work into smaller elements is the “80 hour” rule of thumb. This general guideline states that no single activity or group of activities at the most granular level should produce a single deliverable with more than 80 hours of estimated effort.

This rule of thumb is combined with another guideline: no activity (or group of activities) at the most granular level should exceed the duration of a reporting period. For projects with twice-monthly reporting, the lowest work level should not exceed fifteen days for a single activity. For monthly reporting, that number stretches to thirty days.

Of course, at the end of it all, a large portion of the planning aspects of a WBS fall on the designer or design team. Common sense and experience are both fair indicators and powerful tools when it comes to the planning and subdividing of WBS elements.

These activities, or work packages, that make up the most granular level of planning should fall within the following constraints:

The activities can be realistically estimated with confidence. If the amount of work to be done is so small that it is difficult to estimate, then it should probably be part of a larger task. Additionally, if it doesn’t make practical sense to break down the work package any further, then that alone should be an indicator that the work is already at its most granular level.

The activities must be completable in accordance with the 80-hour and single-reporting-period rules. If an activity doesn’t meet the other two guidelines for determining the smallest work package, then it may still be too high-level and may need to be broken down further. If it does meet those guidelines, then proceed with caution and use common sense before further subdividing work.

The activities must produce a deliverable that is measurable. If the final deliverable isn’t measurable then the work package is too small to track and should probably not have been subdivided so far.

The Bottom Line

The bottom line is that a work breakdown structure is a key visibility and planning document that fills an important role in the successful administration and execution of a project. Because this document is so important, it is equally important that it is constructed correctly and that certain design guidelines and rules are followed for maximum usefulness and application.


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