Before scheduling is done, a Work Breakdown Structure (WBS) should be developed. To illustrate how a schedule is constructed from a WBS, we consider a simple job of maintaining the yard around a home. The WBS is shown in Figure 1.
Figure 1. Work breakdown structure for yard project.
There are two ways to develop a schedule. One is to begin at the end and work back until you arrive at the beginning. The second method is to start at the beginning and work toward the end. Usually, it is easiest to start at the beginning.
This small project might be thought of as having three phases: preparation, execution, and cleanup. There are three preparation tasks: pick up trash, put gas in equipment, and get out hedge clipper. The cleanup tasks include bagging grass, bundling clippings, and hauling trash to the dump.
There is a basic rule of scheduling—to diagram what is logically possible, then deal with resource limitations. For a yard project, if no one is helping me, then there really can be no parallel paths in getting the job done i.e. two tasks cannot take place at the same time. On the other hand, if I can enlist help from the family or neighborhood youth, then parallel paths are possible. The rule I suggest is that you go ahead and schedule as if it were possible to get help. This is especially important to remember in a work setting, or you will never get a schedule put together. You will be worrying about who will be available to do the work and end up in analysis paralysis.
The next step is to figure out how long it will take to do the job. Time estimates for each task are made by using history—remembering how long each activity has taken in the past. Remember, though, that the estimate is valid only for the individual who is going to do the task. If my daughter, who is sixteen, does the lawn mowing using a push mower, it will probably take less time than if my son, who is only twelve, tackles the job.
The first step is to create a work breakdown table. The table will show the name of each task (usually each task is also given a letter code), the duration of the task and what events must be completed before the task can begin. The list of preceding tasks are only those that immediately precede an individual task, not a list of all the tasks that occurred earlier. As an example for the lawn case a work breakdown table would appear as follows:
Pick up trash
Put gas in mower
Get hedge clippers
A and B
Mow front yard
A and B
A and B
D, H, F and G
D, H, F and G
I and J
A network diagram can be drawn from the information in the work breakdown table. D is the task duration and arrows indicate precedence.
Once a suitable network has been drawn, with durations assigned to all activities, ...