The Schedule Network
As described in the opening of this chapter, the schedule network captures the logic of the project, reflecting in its structure the dependencies and relationships among tasks on the WBS. The network we will concern ourselves with is commonly referred to as the PDM. In a PDM network, the tasks of the WBS are represented as nodes or rectangles, and arrow-pointed links relate one task to another. In fact, we can think of the components of a network as consisting of building blocks.
Network Building Blocks
We will use the components shown in Figure 7-1 as the building blocks in our schedule network. As seen there, we have the simple task, the simple path consisting of at least two tasks in tandem, the parallel paths consisting of two or more simple paths joined by a milestone, links between tasks, and milestones that begin, end, and join paths. To simplify illustrations, the milestones are often omitted and are simply implied by the logic of the network. For instance, as shown in Figure 7-2, we see the task itself serving as the starting, ending, and joining mechanism of tasks.
Figure 7-1: Schedule Building Blocks.
Figure 7-2: Milestone Simplifications.
There are certain characteristics that are applied to each schedule task, represented by a rectangle in our network building blocks. These task characteristics are:
Every task has a specific beginning and a specific ending, thereby allowing for a specific duration (ending date minus beginning date) measured in some unit of time (for instance, hours, days, weeks, or months). Rarely would a schedule task be shown in years because the year is too coarse a measure for good project planning.
Every task has some effort applied to it. Effort is measured in the hours spent by a "full-time equivalent" (FTE) working on the task. By example, if the effort on a task is 50 hours, and a FTE is 40 hours, then there is 1.25 FTE applied to the task. If the task duration is 25 hours, then the 50 hours of effort must be accomplished in 25 hours of calendar time, requiring 2.5 FTE. Thus, we have the following equations:
FTE applied to task = (Effort/Duration) * (Effort/Hours per FTE)
FTE applied to task = (50/25) * (50/40) = 2,500/1,000 = 2.5
Every task has not only a specific beginning or ending, but also each task as an "earliest" or "latest" beginning or ending. The idea of earliest and latest leads to the ideas of float and critical path, which will be discussed in detail subsequently. Suffice it to say that the difference in "earliest" and "latest" is "float" and that tasks on the critical path have a float of precisely 0.
Estimating Duration and Effort
We can easily see that the significant metrics in every schedule network are the task durations and the task efforts. These two metrics drive almost all of the calculations, except where paths merge. We address the merge points in subsequent paragraphs. Now as a practical matter, when doing networks for some tasks it is more obvious and easier to apply the estimating ideas discussed in other chapters to the effort and let the duration be dependent on the effort and the number of FTE that can be applied. In other situations just the opposite is true: you have an idea of duration and FTE and the effort simply is derived by applying the equations we described above.
Most network software tools allow for setting defaults for effort-driven or duration-driven attributes for the whole project, or these attributes can be set task by task. For a very complex schedule, setting effort-driven or duration-driven attributes task by task can be very tedious indeed. Perhaps the best practical advice that can be given is to select the driver you are most comfortable with, and make selective adjustments on those tasks that are necessary. Consider this idea however: duration estimating ties your network directly to your program milestones. When a duration-driven network is developed, the ending dates or overall length of the network will fall on actual calendar dates. You will be able to see immediately if there is an inherent risk in the project network and the program milestones.
Perhaps the most important concept is the danger of using single-point estimates in durations and efforts. The PERT network was the first network system to recognize that the expected value is the best single estimate in the face of uncertainty, and therefore the expected value of the duration should be the number used in network calculations. The BETA distribution was selected for the PERT chart system and the two variables "alpha" and "beta" were picked to form a BETA curve with the asymmetry emphasizing the most pessimistic value.  Although the critical path method (CPM) to be discussed below started out using single-point estimates, in point of fact more often than not a three-point estimate is made, sometimes using the BETA curve and sometimes using the Triangular distribution. In effect, using three-point estimates in the CPM network makes such a CPM diagram little different from the PERT diagram.
More information on the BETA "alpha" and "beta" parameters is provided in Chapter 2.