CPM & PERT - Bar charts and milestone charts

2. Bar charts and milestone charts 

a) Introduction

• Bar charts were introduced by Henery Gantt around 1900 AD. 

• Bar charts is a pictorial representation in two dimensions of a project, by breaking the project down into a number of manageable activities for planning and control, which are shown on one dimension and the durations assigned to these activities are shown on the other axes.

• Bar charts were later modified to yield the milestone charts, while the bar chart represents the activities, a mile stone chart represents the events, which mark either the beginning or the end of an activity.

• The bars of the bar chart are broken into a number of pieces, each one of which represent an identifiable major event. 

• Each event is a point in time which the management has identified as important reference point during the completion of the project.

b) Limitations

• Lack of degree of details

1. On bar chart, only major activities are shown. If too many activities are separately shown, it becomes clumsy. Due to this, bar charts are not very useful for big projects.
2. A particular activity, whether big or small, is shown by one bar, without any details of sub-activities contained in it. These sub-activities cannot be separated out. Due to this, effective control over the activities cannot be achieved.
3. The information of these sub-activities can be shown effectively by marking stages, called mile-stones on the activity bar, using small arrows.

• Review of project progress

1. A bar chart does not show the progress of work and hence cannot be used as a control device.
2. For proper control of the project, information of the progress made at a particular instant of time should be available. Controlling is essential for re-scheduling the remaining activities. 
3. However, an existing bar chart can be modified to depict the progress made, by showing the progress of each activity, by hatched lines along the corresponding bar of the activity. Generally, hatching is done in half the width of the bar.

• Activity inter-relationships

By merely depicting the activities by parallel lines, the inter-relationships between the activities cannot be clearly depicted. One cannot draw the conclusion that if two activities are scheduled for simultaneous or overlapping times, they are inter-dependent or completely independent.

• Time uncertainties

1. Bar charts are not at all useful in those projects where there are uncertainties in determination or estimation of time required for the completion of various activities. 
2. Because of uncertainties in time determinations in these projects, some of the activities may require rescheduling. Such rescheduling flexibility cannot be reflected in the bar chart diagrams.