Work breakdown structure is the foundation on which all further planning including cost and schedule are based on. Without a work breakdown structure, accurate estimation of cost and time is not possible. Once the scope of the project is defined, the next logical step is to decompose the scope of the project into a Work Breakdown Structure (WBS). The steps involved in developing the work breakdown structure are as follows;
- Decompose the scope of work into a Product Breakdown Structure (PBS)
- Super-impose the Product Breakdown Structure (PBS) with the work required to be performed to build that component
- Product breakdown structure superimposed with the work to be performed is the work break down structure.
- Work breakdown structures can be decomposed to several levels. By the time work breakdown reaches levels 3,4,5, it will be granular enough to be used for estimating cost and time.
High level WBS
WBS decomposed further
- The lowest level in the WBS is known as the work packages. There will not be any sub-levels below the work package.
- There is a general rule of thumb which states that work packages should contain work which can be completed between 8 hours and 80 hours. We call it as the 8-80 rule. For smaller projects, the affinity of the work packages will be towards 8 hours and for large projects the affinity of the work packages must be towards 80 hours. For very large projects, one may have work packages bigger than 80 hours chunks of work.
- Work package sizing is very important for the effective monitoring and controlling of projects. For very small projects if we have very large work packages, there will not be any status change for a very long time interval where as if for large projects if we have very small work packages, then we end up doing micro management.
- The 8-80 rule comes in handy to understand whether the work breakdown structure is decomposed enough or not.
- Every work package is supplemented with a WBS dictionary, which contains;
- Description of the work to be performed
- Successors and predecessors
- Quality standards to be followed
- Reference to engineering drawings etc
- The WBS dictionary provides sufficient information sufficient enough for the crew on the field to do it right the first time.
- The WBS should cover the entire scope of the project. We call this as the 100 percent rule. The scope which is not captured in the WBS will not be there in the final product. Hence it is important to ensure that the work breakdown structure captures the entire scope of work.
- WBS can be represented as a tree structure or as a list structure
Tree structure
List structure
WBS dictionary
Once the Work breakdown structure is approved, the WBS along with the scope document will become the scope baseline of the project, which will become the basis for all further planning.
Development of the work breakdown strategy is closely linked to the project execution strategy. For example, if the project strategy is to execute the project using a waterfall model (traditional model) with teams of specific skill sets across phases, then having the grouping of requirements, design, construction, testing , release at the level#1 of the WBS will suit the strategy better. If the project strategy revolves around outsourcing many components, then grouping the work packages component wise will be more suitable. If the project strategy involves use of Advanced Work Packaging (AWP), then the path of construction should take precedence over other factors.
Apart from the project execution strategy, how one want to monitor and control the cost of the project also plays a major role while deciding on the structure of the work breakdown structure.
Key checkpoints for Work Breakdown Structures
- 100% rule – Do the sum total of all the work packages cover the complete scope of the project
- Do the WBS structure support the project’s execution strategy
- 8-80 rule – Are the work packages sized and grouped correctly for effective monitoring and controlling
- Are they grouped in such a way that effective allocation of work is feasible
