ᎏᎏ X CA/U ⫻ X CO/EXCA/E/XCA/U
4.4 Gantt Chart
The most widely used management tool for project scheduling and control is a version of the bar chart developed during World War I by Henry L. Gantt. The Gantt chart, as it is called, enumerates the activ- ities to be performed on the vertical axis and their corresponding duration on the horizontal axis. It is possible to schedule activities by either early-start or late-start logic. In the early-start approach, each activity is initiated as early as possible without violating the precedence relations. In the late-start approach, each activity is delayed as much as possible as long as the earliest finish time of the project is not compromised.
A range of schedules is generated on the Gantt chart when a combination of early and late starts is applied. The early-start schedule is performed first and yields the earliest finish time of the project. That time is then used as the required finish time for the late-start schedule.Figure 4.11 depicts the early-start Gantt chart schedule for the example above. The bars denote the activities; their location with respect to the time axis indicates the time over which the corresponding activity is performed. For example, activ- ity D can start only after activities A and B finish, which happens at the end of week 5. A direct output of this schedule is the earliest finish time for the project (22 weeks for the example).
On the basis of the earliest finish time, the late-start schedule can be generated. This is done by shifting each activity to the right as much as possible while still starting the project at time zero and completing it
TABLE 4.2 Data for Example Project
Activity Immediate Predecessors Duration (weeks)
A – 5
B – 3
C A 8
D A,B 7
E – 7
F C,E,D 4
G F 5
in 22 weeks. The resultant schedule is depicted in Figure 4.12. The difference between the start (or the fin- ish) times of an activity on the two schedules is called the slack (or float) of the activity. Activities that do not have any slack are denoted by a shaded bar and are termed critical. The sequence of critical activities connecting the start and end points of the project is known as the critical path, which logically turns out to be the longest path in the network. A delay in any activity along the critical path delays the entire proj- ect. Put another way, the sum of durations for critical activities represents the shortest possible time to com- plete the project.
Gantt charts are simple to generate and interpret. In the construction, there should be a one-to-one correspondence between the listed tasks and the WBS and its numbering scheme. As shown in Figure 4.13, which depicts the Gantt chart for the microcomputer development project, a separate column can be added for this purpose. In fact, the schedule should not contain any tasks that do not appear in the WBS. Often, however, the Gantt chart includes milestones such as project kickoff and design review, which are listed along with the tasks.
In addition to showing the critical path, Gantt charts can be modified to indicate project and activity status. In Figure 4.13, a bold border is used to identify a critical activity, and a shaded area indicates the approximate completion status at the August review. Accordingly, we see that tasks 2, 5, and 8 are critical, falling on the longest path. Task 2 is 100% complete, task 4 65% complete, and task 7 55% complete; tasks 5, 6, and 8 have not yet been started.
Gantt charts can be modified further to show budget status by adding a column that lists planned and actual expenditures for each task. Many variations of the original bar graph have been developed to provide more detailed information for the project manager. One commonly used variation that replaces the bars with lines and adds triangles to indicate project status and revision points is shown in Figure 4.14. To explain the features, let us examine task 2, equipment design. According to the code given in the lower left-hand cor- ner of the figure, this task was rescheduled three times, finally starting in February, and finishing at the end of June. Note the two rescheduled start milestones and the two rescheduled finish milestones.
The problem with adding features to the bar graph is that they detract from the clarity and simplicity of the basic form. Nevertheless, the additional information conveyed to the user may offset the additional effort
0 5 10 15 20
A
B C
D E
F G
Activity
Week FIGURE 4.11 Gantt chart for an early-start schedule.
required in generating and interpreting the data. A common modification of the analysis is the case when a milestone has a contractual due date. Consider, for example, activity 8 (WBS No. 5.0) in Figure 4.14. If man- agement decides that the required due date for the termination of this activity is the end of February (instead of the end of January), then a slack of 1 month will be added to each activity in the project. If, however, the due date of activity 8 is the end of December, then the schedule in Figure 4.14 is no longer fea- sible because the sequence of activities 2, 5, and 8 (the critical sequence) cannot be completed by the end of December.Section 4.12 contains a discussion related to scheduling conflicts and their management.
0 5 10 15 20
A
B C
D E F G
Activity
Week FIGURE 4.12 Gantt chart for a late-start schedule.
No. Task/Milestone WBS
no.
1 2 3 4 5 6 7 8
Project kickoff Equipment design Critical design review Prototype fabrication Test and integration Opers. and maintenance Marketing
Transition to manufacturing
— 1.0
— 2.0 2.2 3.0 4.0 5.0
Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb
Master schedule
Review date FIGURE 4.13 Gantt chart for the microcomputer development example.
The major limitation of bar-graph schedules is their inability to show task dependencies and time- resource trade-offs. Network techniques are often used in parallel with Gantt charts to compensate for these shortcomings.