Gantt Chart
Henry
Gantt’s name is attached to a family of widely used scheduling
charts. A few examples appear in Figure A. In the basic Gantt chart
form, much like Figure A.1, vertical divisions represent time, and horizontal
rows, the jobs or resources to be scheduled. Lines, bars, brackets,
shading, and other devices mark the start, duration, and end of a scheduled
identity. The purpose of the charts, as with any visual aid, is to clarify,
improve comprehension, and serve as a focus for discussion.
The charts in Viewgraph
1 are for scheduling three (3) different resource types: equipment,
space, and employees. Each also identifies the jobs to be performed
by the resources. Note too that each is a services example. While Gantt’s
original chart was for the control of repetitive manufacturing, today
simpler forms of Gantt charts are more widely used in services, where
routings are short and queues have few chances to form.
When should we use Business Gantt Charts?
In good production, Gantt charts may be usable if:
- There are many work centers. With many work centers, a carefully
developed Gantt display of schedules tends to be a piece of gross
fiction, because queuing effects make lead times unpredictable. Keeping
the chart up-to-date under such conditions would be time-consuming
and pointless.
- Job times are long-days or weeks rather than hours. One example
is a construction project. Drywallers, painters, cement crews, roofers,
and so on, may each spend several days or weeks at a work site. With
such a long job time, a schedule on a Gantt chart will hold still
and not become instantly out-of date as it would with very short jobs.
- Job routings are short. In parts manufacturing, routings can be
long. A single job may pass through 5,10, or even 15 work centers,
with unpredictable queue time at each stop. With so much unpredictability,
the Gantt schedule is not believable and thus not worth displaying.
Sometimes a Gantt chart is used for both scheduling and schedule control.
This is especially the case in renovation or maintenance work. Figure
B shows a Gantt control chart for renovation work. Part 1 is an initial
schedule for three (3) crews. An arrow at the top of each chart identifies
the current day.
Viewgraph
2.1 shows progress after one day. The shading indicates amount of
work done, which probably was scheduled by the crew chief, in percent
of completion. Two-thirds of the first paint job was scheduled for Monday,
but the paint crew got the whole job done that day. While the paint
crew is one half-day ahead of schedule, drywall is one-quarter day behind.
Carpentry did Monday’s scheduled work on Monday and is on schedule.
Viewgraph
2.3 for Tuesday shows painting falling behind, drywall on schedule,
and carpentry ahead.
The visual display offered by Gantt charts is a plus. But when things
get complicated, for example, when there are many jobs, many routings,
many work centers, and so forth, visual charts must give way to number-
and word-based schedules. Also, while Gantt chart may be constructed
from a need date backward or from a start date forward, backward scheduling
is more likely to be the case in complex job settings.
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