The WBS is decomposed down to the work package level. A work package is the lowest level in the WBS, and is the point at which the cost and schedule for the work can be reliably estimated.1
A question to be answered in the design of any WBS is when to stop dividing work into smaller elements. If a WBS terminal elements are defined too broadly, it may not be possible to track project performance effectively. If a WBS terminal elements are too granular, it may be inefficient to keep track of so many terminal elements, especially if the planned work is in the distant future. A satisfactory tradeoff may be found in the concept of progressive elaboration which allows WBS details to be progressively refined before work begins on an element of work.
One form of progressive elaboration in large projects is called rolling wave planning which establishes a regular time schedule for progressive elaboration. In reality, an effective limit of WBS granularity may be reached when it is no longer possible to define planned outcomes, and the only details remaining are actions. Unless these actions can be defined to adhere to the 100% Rule, the WBS should not be further subdivided.
The 4% Rule of Decomposition. Gary Heerkens2 suggests a 4% Rule for decomposing a WBS. With this rule a WBS is adequately decomposed when the lowest element is about 4% of the total project. For a 26-week schedule, the lowest element should be about one week. For a $2.6M project, the lowest level should be about $104K.
The 40-Hour Rule of Decomposition. Another rule-of-thumb for determining how far down a WBS should be decomposed is called the “40-Hour Rule.” Generally, when a project has been decomposed down to an element that has about 40 hours of allocated direct labor, there is no need to decompose further. The 40-Hour Rule is based on a 40-hour work week. Because of this, most WBS diagrams are not symmetrical, and some legs may go down to different WBS levels.
It is common for WBS elements to be numbered sequentially to reveal the hierarchical structure. For example, “1267.1.1, Requirements Definition,” identifies this item as a Level 3 WBS element, since there are three numbers separated by decimal points. This coding scheme also helps WBS elements to be recognized in any written context.
MICHAEL D. TAYLOR, M.S. in systems management, B.S. in electrical engineering, has more than 30 years of project, outsourcing, and engineering experience. He is principal of Systems Management Services, and has conducted project management training at the University of California, Santa Cruz Extension in their PPM Certificate program for over 13 years, and at companies such as Sun Microsystems, GTE, Siemens, TRW, Loral, Santa Clara Valley Water District, and Inprise. He also taught courses in the UCSC Extension Leadership and Management Program (LAMP), and was a guest speaker at the 2001 Santa Cruz Technology Symposium. His website is www.projectmgt.com.