This method, which was introduced in 1917, is the oldest and the most extensively used method for production planning, scheduling and control. The Gantt chart shows the relationship between different activities over a time span. Time frame, expressed either in terms of hours, days, weeks or months is shown on the horizontal or X-axis and activities are plotted against the Y-axis.
The time frame or time scale would depend on the nature of operations and activities, which may be determined by the previous experience or an approximation based on which activities may be scheduled and monitored.
The charts may be in the form of any of the following:
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(a) Scheduling or progress charts, which show the sequence of job progress
(b) Load charts which show the work assigned to a work group or allocated to machines
(c) Record charts which track the actual time spent and delays, if any
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Gantt charts need to be updated at regular intervals, for instance, when a work is delayed at the start or when work continues beyond its time schedule or if the progress of work is not as per the actual plan. If unforeseen eventualities occur, corrective actions may have to be taken, and this will also need corresponding changes in Gantt charts.
A Gantt chart is a matrix which lists on the vertical axis all the tasks to be performed. Each row contains a single task identification which usually consists of a number and name. The horizontal axis is headed by columns indicating estimated task duration, skill level needed to perform the task and the name of the person assigned to the task, followed by one column for each period in the project’s duration. Each period may be expressed in hours, days, weeks, months and other time units. In some cases it may be necessary to label the period columns as period 1, period 2 and so on.
The graphical portion of the Gantt chart consists of a horizontal bar for each task connecting the period start and period ending columns. A set of markers is usually used to indicate the estimated and the actual start and end. Each bar on a separate line and the name of each person assigned to the task is on a separate line. In many cases, when this type of project plan is used, a blank row is left between tasks.
When the project is under way, this row is used to indicate progress, indicated by a second bar which starts in the period column when the task is actually started and continues until the task is actually completed. Comparison between the estimated start and end and the actual start and end should indicate project status on a task-by-task basis.
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Variants of this method include a lower chart which shows personnel allocations on a person-by-person basis. For this section, the vertical axis contains the number of people assigned to the project, and the columns indicating task duration are left blank, as is the column indicating persons assigned.
The graphics consist of the same bar notation as in the upper chart indicates that the person is working on a task. The value of this lower chart is evident when it shows the slack time for the project personnel, i.e., times when they are not actually working on any project.
The advantages and disadvantages of Gantt charts are as below:
Advantages:
1. This is a simple and very inexpensive method and can be developed even by supervisory staff with some amount of training
2. These charts clearly show the decided time and work schedules for every job
3. Monitoring and control are easier and can be done within a minimum time frame and at the lowest cost
4. These charts can be changed and updated quickly at a lower cost
5. There is no need to develop the customized Gantt chart boards as the standard chart boards are available in the market
Disadvantages:
In spite of the above-mentioned advantages, there are certain disadvantages.
1. They do not show job interrelationships and interdependence
2. Cost implications cannot be shown
3. With these charts, it is not possible to depict other alternatives for project completion
4. The shape and form of Gantt charts can differ according to the nature of the requirement. Shown below is a very simple Gantt load chart:
The above chart shows that Machine-A has been loaded till the fifth for a given job, and Machine-B has been loaded till the first half of the fourth week and so on.