THE 100 PERCENT RULE

The 100 percent rule is the most important criterion in developing a WBS and in evaluating the decomposition logic. It is formulated as follows:

The next level decomposition of a WBS element (child level) must represent 100 percent of the work applicable to the next higher (parent) element. The rule can be further explained using Figure 2-1.

FIGURE 2-1 Garage Project Work Breakdown Structure

In Figure 2-1, the 100 percent rule means that the sum of the work involved in “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. In a top-down subdivision, most planners would automatically follow the 100 percent rule, at least to Level 2. However, the rule applies at all levels and in all types of WBSs.

At the next level, the 100 percent rule means that the work in the “Garage” element consists of work related to “Materials,” “Foundation,” “Walls,” “Roof,” and “Utilities.” That is, there is no work on the garage itself that does not fit within one of these elements. Turning back to Figure 1-6, 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 encompass all 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 must always ask if all the work is accounted for. Depending on the specific project, there could be a requirement for a “Wall Inspection” that would also be a Level 4 element.

It’s important to note that the primary reason to have a WBS is to ensure that all the work packages and activities that must be accomplished for a successful project are identified. The 100 percent rule also applies at the activity level: The work represented by the activities in each work package must add up to 100 percent of the work necessary to complete the work package. In this manner, the project manager can be reasonably assured that all the work necessary for successful completion of the project has been identified, planned, and scheduled.

The 100 percent rule is an important rule. It helps the person developing the WBS to constantly question himself or herself on the breadth and depth of his or her understanding of the project work. 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 ascertain that their specialty is properly included in the work, and they can contribute to making sure the WBS is complete and as accurate as possible. On manufacturing projects, for example, it is useful to elicit input from manufacturing engineers or tooling engineers for the identification of probable subassemblies. In software projects, it is useful to elicit input from systems analysts, programmers, database specialists, and the like. For our garage example, input from an experienced carpenter or garage-builder would be useful.

Not all WBSs are based on products. The types of WBSs are Product WBS, Service WBS, and Results 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 “Systems Engineering,” “Research,” or “Meetings.”

The use of bottom-up cost estimating, that is, estimating the costs of every activity or 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.

One of the common ways to develop a WBS is from the bottom up, and this approach is especially useful if the output product of the project is a service. All the project activities are first listed in a brainstorming session and then grouped into logical work packages or lower-level WBS elements. These are in turn summarized into higher-level elements. The 100 percent rule is followed at each level, and this question is asked at each level: “Does the sum of the work represented by the child elements equal 100 percent of the effort summarized in each parent element?” Or this question is asked: “Is any work missing?” Experience has indicated that asking these questions invariably results in additional activities being added, and several iterations of the WBS may be performed until a sound WBS is developed.

The importance of the 100 percent rule to the proper functioning of the WBS as a framework for planning cannot be overstated. If the decomposition at each level follows the 100 percent rule down to the activities, then 100 percent of the relevant activities will have been identified when it is time to prepare the project schedule. Therefore, 100 percent of the costs or resource requirements will be successfully identified in the planning phase.