Bar Gantt Chart

The most common type of display is the bar or Gantt chart, named for Henry Gantt, who first utilized this procedure in the early 1900s. The bar chart is a means of displaying simple activities or events plotted against time or dollars. An activity represents the amount of work required to proceed from one point in time to another. Events are described as either the starting or ending point for either one or several activities.

Bar charts are most commonly used for exhibiting program progress or defining specific work required to accomplish an objective. Bar charts often include such items as listings of activities, activity duration, schedule dates, and progress-to-date. Figure 13-1 shows nine activities required to start up a production line for a new product. Each bar in the figure represents a single activity. Figure 13-1 is a typical bar chart that would be developed by the program office at program inception.

Bar charts are advantageous in that they are simple to understand and easy to change. They are the simplest and least complex means of portraying progress (or the lack of it) and can easily be expanded to identify specific elements that may be either behind or ahead of schedule.

Bar charts provide only a vague description of how the entire program or project reacts as a system. There are three major discrepancies in the use of a bar chart. First, bar charts do not show the interdependencies of the activities, and therefore do not represent a "network" of activities. This relationship between activities is crucial for controlling program costs. Without this relationship, bar charts have little predictive value. For example, does the long-lead procurement activity in Figure 13-1 require that the contract be signed before procurement can begin? Can the manufacturing plans be written without the material specifications activity being completed? The second major discrepancy is that the bar chart cannot show the results of either an early or a late start

Page 728

ACTIVITY

CONTRACT NEGOTIATED

a

CONTRACT SiGNED

LONG LEAD PROCUREMENT

MANUFACTURING SCHEDULES

i i i

BILL OF MATERIALS

SHORT LEAD PROCUREMENT

MATERIAL SPECIFICATIONS

□ □

i

MANUFACTURING PLANS

1

START-UP

n

0

WEEKS AFTER GO-AHCAD

Figure 13-1. Bar chart for single activities.

in activities. How will a slippage of the manufacturing schedules activity in Figure 13-1 affect the completion date of the program? Can the manufacturing schedules activity begin two weeks later than shown and still serve as an input to the bill of materials activity? What will be the result of a crash program to complete activities in sixteen weeks after go-ahead instead of the originally planned nineteen weeks? Bar charts do not reflect true project status because elements behind schedule do not mean that the program or project is behind schedule. The third limitation is that the bar chart does not show the uncertainty involved in performing the activity and, therefore, does not readily admit itself to sensitivity analysis. For instance, what is the shortest time that an activity might take? What is the longest time? What is the average or expected time to activity completion?

Even with these limitations, bar charts do, in fact, serve as a useful tool for program analysis. Even the earliest form of bar chart, as developed by Henry Gantt, still has merit under certain circumstances. Figure 13-2 shows the conventional usage for work scheduled in a production facility for twelve days in January. On Thursday of the first week, the production facility was idle owing to lack of materials. By the end of the workday on Friday of the first week, only 280 out of the planned 300 units were produced. The production line was not available on either Saturday or Sunday, and operations resumed Monday. On Tuesday, the production line was down for repairs and did not resume operations until Thursday. Operations were sporadic on Thursday and Friday, and by the end of the day, only 340 out of a scheduled 400 units were completed. These types of ap-

Team-Fly®

JANUARY

M

T

W

TH

F

S

S

M

T

W

TH

F

WÎW

LEGEND

SYMBOLS

A WORK LOAD SCHEDULED B WORK LOAD COMPLETED

THIN LtNE IDENTIFIES SCHEDULED OPERATIONS

TH1N LIME IDENTIFIES COMPLETED OPERATIONS

TIME PERIOD BLOCKED OFFNOT AVAILABLE FOR PRODUCTION

LETTER

M LACK OF MATERIALS H DO WM FOR REPAIRS

Figure 13-2. Manufacturing schedule for Model B-63 flanges.

plications are commonly used for equipment layout and usage, department loading, and progress tracking.1

Some of the limitations of bar charts can be overcome by combining single activities as shown in Figure 13-3. The weakness in this method is that the numbers representing each of the activities do not indicate whether this is the beginning or the end of the activity. Therefore, the numbers should represent events rather than activities, together with proper identification. As before, no distinction is made as to whether even 2 must be completed prior to the start of event 3 or event 4. The chart also fails to define clearly the relationship between the multiple activities on a single bar. For example, must event 3 be completed prior to event 5? Often, combined activity bar charts can be converted to milestone bar charts by placing small triangles at strategic locations in the bars to indicate completion of certain milestones within each activity or grouping of activities, as shown in Figure 13-4. The exact definition of a milestone differs from company to company, but usually implies some point where major activity either begins or ends, or cost data become critical.

Bar charts can be converted to partial interrelationship charts by indicating (with arrows) the order in which activities must be performed. Figure 13-5 represents the partial interrelationship of the activities in Figures 13-1 and 13-3. A full interrelationship schedule is included under the discussion of PERT networks in Chapter 12.

1 A. C. Laufer, Operations Management (Cincinnati: Southwestern Publishing Co., 1975); see pp. 106-108 for examples of Gantt charts and nomenclature.

Figure 13-3. Bar chart for combined activities.

The most common method of presenting data to both in-house management and the customer is through the use of bar charts. Care must be taken not to make the figures overly complex so that more than one interpretation can exist. A great deal of information and color can be included in bar charts. Figure 13-6 shows a grouped bar chart for comparison of three projects performed during different years. Care must be taken when using different shading techniques that each area is easily definable and that no major contrast between shaded areas exists, except for possibly the current project. When grouped bars appear on one chart, nonshaded bars should be avoided. Each bar should have some sort of shading, whether it be cross-hatched or color-coded.

Contrasting shaded to nonshaded areas is normally used for comparing projected progress to actual progress as shown in Figure 13-7. The tracking date line indicates the time when the cost data/performance data was analyzed. Project 1 is behind schedule, project 2 is ahead of schedule, and project 3 is on target.

tf?

WEEKS AFTER GO AHEAD

MtLESTONES

1

PURCHASE ORDER RELEASE

2

INVOICES RECEIVED

3

MATERIAL RECEIVED

Figure 13-4. Bar/milestone chart.

Figure 13-4. Bar/milestone chart.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment