THE 100 PERCENT RULE

The 100 percent rule is the most important concept in developing a WBS and in evaluating the decomposition logic:

Starting with a specific WBS element (it can be the single block at Level 1), the sum of the elements at the next level of decomposition (child level) must represent 100 percent of the work applicable to the next higher (parent) element.

In Figure 2-1, the 100 percent rule means that the sum of the work involved in Level 2: “landscaped grounds,” “garage,” and “project management” equals 100 percent of the work to be performed in the garage project. There is no project activity that does not fit within one of these categories. The rule applies not only at Level 2, but at all levels where there are parent/child relationships and in all types of WBSs.

FIGURE 2-1
Garage Project Work Breakdown Structure

At Level 3, the 100 percent rule means that the work in the garage element consists of work related to foundation, walls, roof, and utilities. There is no work on the garage itself that does not fit within one of these elements. Turning back to Figures 1-6 and 1-7, the walls element would be represented 100 percent by the walls and siding, windows, garage door, service door, and assembly elements at Level 4. Again, these five elements make up 100 percent of the work on the walls. Note that the assembly element is necessary to account for the work of installing or integrating the work of the other independent elements. This is an important element that is frequently forgotten. The person developing the WBS (or the subject-matter experts) must always ensure that all the work is accounted for.

The primary reason to have a WBS is to ensure that all the work packages and activities that must be accomplished for a project to be successful are identified. The 100 percent rule also applies at the work package/activity level: The work represented by the activities in each work package (the child elements) must add up to 100 percent of the work necessary to complete the work package (the parent element). In this manner, the project manager can be reasonably certain that all the work necessary for successful completion of the project has been identified and the basis exists for further planning, budgeting, and scheduling.

A recommended practice, and one that is common, is collegial development—or at least collegial review—of the WBS. Subject-matter experts will always try to verify that their specialty is properly included in the work, and they can contribute to making sure the WBS is as complete and accurate as possible. On acquisition projects, for example, it is useful to elicit input from quality-control engineers or engineers from the operational organization to identify probable subassemblies. In software projects, it is useful to elicit input from systems analysts, programmers, and database specialists. For our garage example, input from an experienced carpenter or garage builder would be useful, especially in identifying the lower levels of detail.

Not all WBSs are based on products; the government has many other types of projects. The other two types of WBSs are the service WBS and the result WBS. The rule still applies: The sum of the work in the child elements must equal 100 percent of the work represented by the parent element—even if the parent element is a general term like “preliminary engineering,” “hypothesis test,” or “meetings.”

The use of bottom-up cost estimating—estimating the costs of every activity in each work package and summing the data up the WBS hierarchy into a total project cost—is based on the critical assumption that the WBS is developed by following the 100 percent rule. If the output product of a project is a service, the WBS should be developed from the bottom up or middle out. All the project activities are first listed in no particular order and without regard to level in a brainstorming session and then grouped into logical work packages or lower-level WBS elements. These are in turn organized and summarized into higher-level elements. The 100 percent rule is followed at each level, repeatedly asking, “Does the sum of the work represented by the child elements equal 100 percent of the effort summarized in each parent element?” This question can also take the form, “Is any work missing?” Asking these questions invariably results in the addition of activities. Several iterations of the WBS should be performed until a sound and complete WBS is developed.