Constructing a Network Diagram from a Work Breakdown Schedule

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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:

TASK

DESCRIPTION

PRECEDING EVENT

DURATION

A

Pick up trash

None

15

B

Put gas in mower

None

5

C

Get hedge clippers

None

5

D

Trim weeds

A and B

45

E

Mow front yard

A and B

12

F

Edge sidewalk

A and B

15

G

Trim hedge

C

30

H

Mow backyard

E

30

I

Bag grass

D, H, F and G

30

J

Bundle Trash

D, H, F and G

15

K

Haul Trash

I and J

45

A network diagram can be drawn from the information in the work breakdown table. D is the task duration and arrows indicate precedence.


Scheduling Computations

Once a suitable network has been drawn, with durations assigned to all activities, it is necessary to determine where the longest path is in the network and whether it will meet the target completion date.

First, consider what we want to know about the project. If it starts at some time = zero, we want to know how soon it can be finished. Naturally, in most actual work projects, we have been told when we must be finished; that is, the end date is dictated. Further, the start date for the job is often con­strained for some reason: resources won’t be available, specs won’t be written, or another project won’t be finished until that time. Therefore, scheduling usually means trying to fit the between two fixed points in time. Whatever the case, we want to know how long the project will take to complete; if it won’t fit into the required time frame, then we will have to do something to shorten its duration.

In the simplest form, computations are made for the network on the assumption that activity durations are exactly as specified. However, activity durations are a function of the level of resources applied to the work, and if that level is not actually available when it comes time to do the work, then the scheduled dates for the task cannot be met. It is for this reason that network computations must ultimately be made with resource limitations in mind. Another way to say this is that resource allocation is necessary to determine what kind of schedule is actually achievable. Failure to consider resources almost always leads to a schedule that cannot be met.

The first step in network computations is to determine the schedule and what kind of latitude is available to shift resources to shorten the project’s duration. The ideal situation is one in which unlimited resources are available, so the first computations made for the network are done ignoring resource requirements.

Network Rules and Computation

Three rules are applied to all networks in order to compute network start and finish times.

Rule 1: Before a task can begin, all tasks preceding it must be completed.

Rule 2: Arrows denote which tasks come first.

Rule 3: When two or more activities precede another activity, the earliest time that the final activity can be started is the larger of the durations of the activities preceding it.

Let’s look at an example to clarify what the rules mean.  First, let us examine the boxes in the network diagram below used to calculate the schedule. Each box in the network now contains three pieces of information, not just the duration as in the network shown above.  The box shows:

D= Duration (of the task)

ES= Earliest Start i.e. the earliest time the task can begin, given all preceding tasks are completed

EF= Earliest Finish i.e. the earliest time the task can finish, given the completion of all preceding tasks and the task  itself.

Network diagram for yard project used for schedule calculations.

 

Earliest finish of a task equals the earliest start plus the duration of the task or EF= ES +D.

Consider a single activity in the network, such as picking up trash from the yard (TASK A). It has a duration of fifteen minutes. Assuming that it starts at time = zero, it can finish as early as fifteen minutes later. We therefore can enter fifteen in the cell labeled EF.  Putting gas in the mower and the weed whacker (TASK B) takes only five minutes. The logic of the diagram says that both of these tasks must be completed before we can begin trimming weeds, cutting the front grass, and edging the sidewalk (TASKS D,E and F). The cleanup task takes fifteen minutes, whereas the gas activity takes only five minutes. How soon can the following activities start? Not until the cleanup has been finished, since it is the longest of the preceding activities (rule 3) In fact, then, the Early Finish for cleanup becomes the Early Start for the next three tasks. It will always be true that the latest Early Finish for the earlier tasks becomes the Early Start for subsequent tasks. That is, the longest path deter­mines how early subsequent tasks can start.

Following this rule, we can fill in Earliest Start times for each task in the network diagram. This diagram shows that the project will take a total of 165 minutes to complete, if all work is conducted exactly as shown.  Computer programs do exactly this computation and also convert the times to calendar dates, making quick work of the computations.

The time determined for the end or final task (TASK K) is the earliest finish for the whole project in working time.

 The diagram for the PH 453 case study is considerably simpler than this example, but you still must apply the three rules to draw the diagram and figure out the earliest time at which the surveillance project can be completed.


Network Diagram Case Study last updated April 20, 2004 (epowell)