The activity's identity comes from the work breakdown structure. When the WBS is completed, the bottom of the work breakdown structure defines the individual pieces of work that are necessary to complete the project. These individual pieces of work are the same items that become the activities in the project schedule. One of the things that is done in the development of the WBS is to check that each of the activities has inputs for the work required. Each output from an activity is used by another activity or is required as a part of a project deliverable. Since an activity requires something from another activity, which is that activity's output, both activities must be done in sequence or at least partly so. An activity that is not complete but delivers enough of its outputs to the dependent activity to allow it to start can overlap the dependent activity. Taking advantage of this and scheduling activities to be done in parallel when they would have otherwise been done in sequence is called fast tracking.
Dependencies can be categorized as mandatory, discretionary, or external, and they can be restricted by constraints and assumptions. Mandatory dependencies are those that are required as part of the nature of the work. These dependencies are sometimes called ''hard'' dependencies. For example, the walls of a house cannot be built until the foundation is completed. Discretionary dependencies are those that are defined by management. These are preferred ways of doing things and may be determined by past experiences. External dependencies are those that are external to the project. These are all of the inputs that are supplied by anyone or anything outside the project.
Activity on arrow diagramming (AOA) is a network diagramming method that is seldom used today. Still, there are a number of places where these can still be seen (mainly in the PMBOK). Nearly all, if not all, software programs that are available for project management scheduling have stopped using this diagramming method. However, because they are still present in the latest release of the Guide to the PMBOK, we will discuss them here very briefly.
In activity on arrow diagrams the network diagram will always be shown with the activity information on the arrows instead of in the nodes of the diagram. The nodes of an activity on arrow diagram will always be shown as circles. This diagramming nomenclature is always followed.
Each activity in the diagram has two events associated with it. These events are of zero duration and are located at the beginning of the arrow and at the end of the arrow. This means that there are three things associated with each activity in the diagram: the activity description itself, the starting event of the activity, and the ending event of the activity.
The one advantage of this diagramming method is that, since the arrow is a line, the length of the line can be varied in proportion to the duration of the activity. This can be helpful in recognizing the magnitude of the duration of the activity. However, most project management professionals feel that the complexity and difficulty in using this diagramming method is not compensated for by this feature.
Since the arrows in the diagram in Figure 2-1 represent the activities, it is necessary to create dummy activities to show multiple dependencies in the project being represented.
Calculations of schedules using this diagramming method are a bit more compli
Figure 2-1. Activity on arrow diagram.
cated than the activity on node network diagram (Figure 2-2), but the results will be the same.
Once the schedule has been determined it is necessary to communicate it to those who will use it for information about the project schedule. Both of these techniques are good communication techniques and have their place alongside of the network diagram.
The Gantt chart is the familiar bar chart that we have all seen many times. It has horizontal bars, one for each activity. There is a timescale that is usually linear. The bars are drawn proportional to their duration with the start and finish of the activity corresponding to the dates on the timescale. Using project management software it is possible to annotate the bars with color and text information from the database for the project. Care must be taken to limit the amount of data displayed so that clutter does not occur.
Figure 2-2. Activity on node diagram.
Name Duration Slack lead/lag lead/lag
The milestone chart is a simplification of the Gantt chart. In the days before computers, Gantt charts were done on large pieces of paper, usually several large pieces of paper. If each of the projects were to submit the full schedule to the project manager's manager the volume of paper would be overwhelming. To solve this problem, the project managers instead reported milestones. A milestone represents a group of work activities, usually an element in the work breakdown structure. Milestones have a duration of zero. The milestones could all be plotted on one chart for the manager, called the milestone chart.
Milestones can be created in project management software by creating an activity that has a duration of zero. Project management software actually gives us a better choice, and it is done automatically. If the WBS is entered into the software it will automatically create summary activities that show the start and finish dates for the earliest start and the latest scheduled finish date for the activities subordinate to them. If all of the WBS is entered there will be a summarizing hierarchy of all of the elements in the WBS including a project summary bar, which shows the start and finish schedule for the project.
Precedence diagramming is the method currently being used in nearly all of the project management scheduling software available today. This diagram is used to explain the mechanics of scheduling.
Precedence diagrams can be easily recognized. The network diagram will always be shown with the activity information on the nodes instead of on the arrows of the diagram. The nodes of an activity on precedence diagrams will always be shown as rectangles. This diagramming form is always followed.
In its simplest form the diagram contains boxes to indicate the activities in the schedule and arrows connecting them. The boxes can contain any activity information that is desired, and all of the project management scheduling software today has a great deal of flexibility in this regard. Today, all of this is done through the use of computer software for project management scheduling. The software allows you to annotate the boxes in the diagram with nearly any information you desire. Color and symbols can be used effectively to describe the diagram more fully.
The basic information normally included in the precedence diagram boxes is the activity number, description, early start, early finish, late start, late finish, and duration. The arrows connect the activities according to the logic that is required by the project. The arrows indicate the logical order that the activities may be worked on. The logic of the schedule can be considered as two activity pairs at a time. A pair of activities is any two (and only two) activities that are joined by an arrow. The tail (the part without the head) of the arrow indicates the independent activity of the pair, and the head of the arrow indicates the dependent activity. Reading the logic of the diagram is easy if you keep this in mind and always consider the logical relationships of the network two activities at a time.
By saying that a relationship exists between two and only two activities, I do not mean to say that any activity cannot have more than one relationship. An activity might have two or more predecessors, and it might have two or more successors (Figure 2-3). The relationship has only two activities associated with it, however.
Was this article helpful?
What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.