Concept: Work Breakdown Structure


  1. Introduction
  2. Definition
  3. WBS Role
  4. WBS Structure
  5. Large Project WBS
  6. Frequently Asked Questions

1. Introduction

A key strategy of effective planning is to partition the project into manageable chunks that can be individually planned estimated and controlled. The work breakdown structure is a graphical tool that displays the project's statement of work making it easier to understand and communicate. It is employed from the earliest stages of project planning.

2. Definition

Generic WBSThe work breakdown structure (WBS) is a powerful tool for expressing the scope or extent of a project in simple graphic terms. It represents the project in terms of the hierarchy of deliverables and services it will produce. The project is therefore described just as a manufacturer would document the bill of materials breakdown for a washing machine or automobile. The WBS starts with a single box at the top which represents the whole project. The project is then partitioned into its components with lower level boxes.

The WBS supports the principle of management by deliverables providing a map of what is to be produced.

3. WBS Role

The role of the WBS is to:

Note that the WBS provides a simple map of what is to be produced. It does not deal with schedules and therefore has no time dimension. It is however used as entry criteria for schedule development.

4. WBS Structure

Generic WBSFigure 42.1 describes a typical high level WBS structure for a small to medium sized project. The box at the top (The Project) represents the total system and is referred to as WBS level 1. Lower levels which describe project components in increasing detail are numbered 2, 3, 4 and so on.

The concept of WBS level is important as it allows you to designate the level of detail at which you report cost estimates and project cost performance figures. For example, to effectively manage a large project a senior manager typically needs summary details of cost variance at level 2. By contrast the team leader responsible for developing a level 2 software component needs cost performance reports at level 3.

The partitioning of the project into major components occurs at level 2. Components at this level fall into the following classes:

Project Service. Project services that apply to the entire project and cannot be allocated to a single deliverable item. Examples are, project management and quality management. In the case of design services the overall architectural design of a system is classed as a Project Service as it is used to discover what the components are. The detailed design of a single software component however is allocated to that component.

Hardware Component. A bill of materials breakdown of computer hardware.

Software Component. A bill of materials breakdown of computer software.

5. Large Project WBSLarge Project WBS

Figure 42.2 describes an high level WBS structure for a large project that produces multiple systems. Each system has hardware and software components. Note that this project is described with a 3 level WBS. Therefore, the larger a project becomes the more WBS levels are required to represent it.

The lowest levels of a WBS represent discrete deliverable items against which costs will be tracked and performance measured. For example, say System B's Software Component is a single program developed by two programmers. Typically the project manager will estimate and track the cost of the complete component. He will not be interested in determining the cost of writing subcomponents of this item such as individual subroutines. Therefore, the bottom level of a WBS is determined by the level at which you can realistically estimate costs and control development.

6. Frequently Asked Questions

Q. What do I do with plans? Are planning documents such as project plans classed as WBS elements?
A. They should be if they are major deliverables and/or the organization wants to track their preparation cost. On a small project they could be deemed part of the project management service and not specifically called out as a deliverable.
Q. What about development activities? How can we schedule from a WBS if it describes deliverables only? Where and when do we define the activities?
A. That is the next step. The WBS is the input into activity definition. Now that we have defined what is to be built we can specify how it is to be built by attaching activities to each bottom level WBS element. Most project management tools allow you build a WBS hierarchy. The elements can be anything you want. It's up to you to make sure that the upper level elements are deliverables and the lower level ones are the activities that create the deliverables. If you don't plan this way you will find it impossible to figure out how much an element cost.
Q. Tiny deliverables. What do I do with tiny deliverables like one page test cases? Does the WBS have to document every little thing?
A. No. If it is small it can be viewed as part of a service with no particular deliverable. For example, the test case can be part of the test service. However, if a test case was a major document that took 2 working years to design, it would be viewed as a WBS element.
Q. Think time with no deliverable. What do I do with time spent thinking about general technology problems? This task almost never has a deliverable.
A. Time spent thinking can generally be classified as "technical investigation". Short term technical investigation activities can be part of a design service. If it relates to a specific issue such as the system architecture you should allocate it to a specific deliverable - for example, a system architecture specification.
In general if you are doing a lot of thinking and producing no physical output you should be concerned about your job! Try producing a technical investigation report as a deliverable.

Software Engineering Web
1997 Chambers & Associates Pty Ltd
Module: 42 v1.0 wbs_cncp.htm
Updated: July 02, 2006