The work breakdown structure is the key to successful projects. The work breakdown structure produced a list of the individual pieces of work that must be done to complete a project. These are the building blocks of the project. Each of these represents a portion of the work of the project. Each must be the responsibility of one and only one person on the project team. The person responsible for an individual piece of work is similar to the project manager and is responsible for all that happens in the project regarding that piece of work. That person is responsible for scheduling, cost estimating, time estimating, and of course seeing that the work gets done. Like the project manager, the person responsible may not be required to do all the work. He or she is, however, responsible for seeing that it gets done.
You perhaps have noticed that I have been using the phrase "individual piece of work" to describe the bottom level of the WBS. This is because the Professional Management Institute (PMI) makes a distinction between terms. These individual pieces of work can be referred to as work packages, activities, or tasks. Most project managers would not make a distinction between these three terms, and if they did, they would probably disagree about the meanings of the terms. Most project managers will use the words activity and task interchangeably.
According to the Guide to the PMBOKdefinition of these terms, a work package is the lowest level of the WBS. This means that it is the lowest level that the project manager intends to manage. In a very large project with a hierarchical structure of project managers and subproject managers, there will be managers for the work packages, and each manager will have his or her own work breakdown structure. Eventually a point is reached where cost, resources, and duration define the individual pieces of work. These, according to the Guide to the PMBOK, are called activities. Activities may be further subdivided into tasks. Learning all this may get you a point on the PMP exam, but in this book I will use the words activity and task interchangeably.
In order to determine the project cost accurately enough to be considered the project cost baseline, a bottom up estimate must be made. This estimate must have an accuracy of —5 percent to + 10 percent. This type of estimate will be produced by estimating the cost of each item at the bottom level of the WBS and then summarizing or rolling up the data to the project level.
Bottom up estimates are inherently more accurate because they are a sum of individual elements. Each of the individual elements has a possibility of being over or under the actual cost that will occur. When they are added together, some of the overestimates will cancel out some of the underestimates.
You perhaps have noticed that I have been using the phrase "individual piece of work" to describe the bottom level of the WBS. This is because the Professional Management Institute (PMI) makes a distinction between terms. These individual pieces of work can be referred to as work packages, activities, or tasks. Most project managers would not make a distinction between these three terms, and if they did, they would probably disagree about the meanings of the terms. Most project managers will use the words activity and task interchangeably.
According to the Guide to the PMBOKdefinition of these terms, a work package is the lowest level of the WBS. This means that it is the lowest level that the project manager intends to manage. In a very large project with a hierarchical structure of project managers and subproject managers, there will be managers for the work packages, and each manager will have his or her own work breakdown structure. Eventually a point is reached where cost, resources, and duration define the individual pieces of work. These, according to the Guide to the PMBOK, are called activities. Activities may be further subdivided into tasks. Learning all this may get you a point on the PMP exam, but in this book I will use the words activity and task interchangeably.
In order to determine the project cost accurately enough to be considered the project cost baseline, a bottom up estimate must be made. This estimate must have an accuracy of —5 percent to + 10 percent. This type of estimate will be produced by estimating the cost of each item at the bottom level of the WBS and then summarizing or rolling up the data to the project level.
Bottom up estimates are inherently more accurate because they are a sum of individual elements. Each of the individual elements has a possibility of being over or under the actual cost that will occur. When they are added together, some of the overestimates will cancel out some of the underestimates.