- Sequence: The order in which you perform the activities
- Duration: How long each individual activity takes
To develop a schedule for a small project, you can probably consider the durations and sequential interdependencies in your head. But projects with 15 to 20 activities or more — many of which you can perform at the same time — require an organized method to guide your analysis.
A network diagram’s elementsA network diagram is a flowchart that illustrates the order in which you perform project activities. It’s your project’s test laboratory — it gives you a chance to try out different strategies before performing the work.
No matter how complex your project is, its network diagram has the following three elements: milestones, activities, and durations.
MilestoneA milestone, sometimes called an event, is a significant occurrence in the life of a project. Milestones take no time and consume no resources; they occur instantaneously. Think of them as signposts that signify a point in your trip to project completion. Milestones mark the start or end of one or more activities or the creation of deliverables. Examples of milestones are draft report approved and design begun.
ActivityAn activity (also called a task) is a component of work performed during the course of a project. Activities take time and consume resources; you describe them by using action verbs. Examples of activities are design report and conduct survey.
Make sure you clearly define activities and milestones. The more clearly you define them, the more accurately you can estimate the time and resources needed to perform them, the more easily you can assign them to someone else, and the more meaningful your reporting of schedule progress becomes.
DurationDuration is the total number of work periods completing an activity takes. Several factors can affect duration:
- The amount of work effort (the amount of time a person needs to work full-time on the activity to complete it) required.
- People’s availability to work on the project.
- Whether multiple people can work on the activity at the same time.
- Capacity of non-personnel resources (for example, a computer’s processing speed and the pages per minute that a copier can print) and their availability.
- For example, if your boss spends one hour reading your memo after it sat in her inbox for four days and seven hours, the activity’s duration is five days, even though your boss spends only one hour reading it.
The units of time describe two related but different activity characteristics. Duration is the number of work periods required to perform an activity; work effort is the amount of time a person needs to complete the activity. For example, suppose four people have to work together full time for five days to complete an activity. The activity’s duration is five days. The work effort is 20 person-days (4 people times 5 days).Understanding the basis of a duration estimate helps you figure out ways to reduce it. For example, suppose you estimate that testing a software package requires that it run for 24 hours on a computer. If you can use the computer only six hours in any one day, the duration for your software test is four days. Doubling the number of people working on the test doesn’t reduce the duration to two days, but getting approval to use the computer for 12 hours a day does.
How to draw a network diagramDetermining your project’s end date requires you to choose the dates that each project activity starts and ends and the dates that each milestone is reached. You can determine these dates with the help of a network diagram.
The activity-on-node technique (also called activity-in-box or precedence diagramming method) for drawing a network diagram uses the following three symbols to describe the diagram’s three elements:
- Boxes: Boxes represent activities and milestones. If the duration is 0, it’s a milestone; if it’s greater than 0, it’s an activity. Note that milestone boxes are sometimes highlighted with lines that are bold, double, or otherwise more noticeable.
- Letter t: The letter t represents duration.
- Arrows: Arrows represent the direction work flows from one activity or milestone to the next. Upon completing an activity or reaching a milestone, you can proceed either to a milestone or directly to another activity as indicated by the arrow(s) leaving that box.
Note: If you’ve worked with network diagrams in the past, you may have seen them drawn in another format called activity-on-arrow, also called the classical approach, an arrow diagram, or a PERT chart (see the later section “Improving activity duration estimates” for an explanation of PERT analysis). This format represents milestones with circles and activities with arrows. However, because the activity-on-node technique is the one most used today, all network diagrams in this chapter are drawn in this format.