Gantt charts

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Gantt charts

20.1 Purpose

A Gantt chart is a tool for graphically depicting a schedule. Gantt charts can be used to plan, record, and document the schedule, and to track actual results against the schedule.

20.2 Strengths, weaknesses, and limitations

Gantt charts are easy to create and easy to understand. They are particularly useful for planning relatively small projects because they can often show the entire schedule at a glance.

A Gantt chart is primarily a planning tool. Gantt charts are not as useful for project control because the percent completion depicted by a bar is based on subjective judgment. Also, a Gantt chart does not show the precedence relationships between the tasks. Consequently, project networks (Chapter 21), PERT, and CPM are better for larger projects.

20.3 Inputs and related ideas

Before preparing a Gantt chart the tasks or activities to be performed must be identified and each activity’s duration, start time, and end time estimated. A project network (Chapter 21) is a better tool for scheduling a large project.

20.4 Concepts

On a Gantt chart (Figure 20.1), the activities (or tasks) are listed at the left and time progresses from left to right across the top. In this example, time is shown in days.

Each activity is represented by a horizontal bar. The bar’s left edge indicates when the activity begins, its length corresponds to the activity’s duration, and its right edge shows when the activity ends. Typically, a vertical line identifies the current day. Often, the bars that represent the plan are shown in one color and the actual results are shown in a contrasting color. The result is an easily visualized comparison between the plan and actual performance.

The Gantt chart in Figure 20.1 assumes that all work will be done by one programmer/analyst and one data entry clerk, with the bulk of the work beginning after the customer’s new computer is installed.Figure 20.2 shows a different Gantt chart for the same project. On this schedule, design work begins as soon as the new computer is ordered and a programmer writes the necessary code. Note that the work is completed in significantly less time because more tasks are done in parallel.

20-01
Figure 20.1  A Gantt chart shows the schedule for all a project’s activities at a glance.

20-02
Figure 20.2  On this Gantt chart, several activities are performed in parallel.

20.5 Key terms
Activity —
A task to be completed.
Duration —
The elapsed time required to complete an activity.
Gantt chart —
A chart that shows a project schedule as a series of horizontal lines or bars.
Schedule —
A series of events or activities with estimated completion times or target dates.
20.6 Software

The Gantt charts in this chapter were created using the Visio Timeline Wizard. Most project management software tools (such as Microsoft Project, Primavera’s Suretrack Project Manager, and CA-SuperProject) support Gantt charts.

You can also create a Gantt chart using spreadsheet software such as Excel, Lotus 1-2-3, or Quattro Pro. List the activities in column A, their start times (in days, weeks, months, or other time units from the beginning of the project) in column B, and each activity’s duration in Column C. Plot the data as a horizontal, stacked bar chart and select attributes that make the first bar (the start times) invisible.

20.7 References
1.  Badiru, A. B. and Whitehouse, Computer Tools, Models and Techniques for Project Management, TAB Books, Blue Ridge Summit, PA, 1989.
2.  Davis, W. S., Business Systems Analysis and Design,Wadsworth, Belmont, CA, 1994.
3.  Humphrey, W. S., Managing the Software Process, Addison-Wesley, Reading, MA, 1989.
4.  Roetzheim, W. H., Structured Computer Project Management,Prentice-Hall, Englewood Cliffs, NJ, 1988.
5.  Weinberg, G. M. and Weinberg, D., General Principles of Systems Design,Dorset House, New York, 1988.

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