Rules Of Drawing Network Diagram

Network Diagram Representation

In a network representation of a project few definitions are used

1. Activity

Any individual operation which makes use of resources and has an end and a starting is called as activity. An arrow is generally used to characterize an activity with its head representing the direction of progress in the project. These are divided into four classes

Predecessor activity - Activities that ought to be completed immediately previous to the start of another activity are known as predecessor activities.
Successor activity - These are those activities that cannot be started until one or more of other activities are finished but immediately succeed them.
Concurrent activity - These are activities which can be achieved concurrently. It may be noted that an activity can be a successor or a predecessor for an event or it may be concurrent with one or more activities.
Dummy activity - An activity which does not use any type of resource but simply depicts the technological dependence is known as dummy activity.

The dummy activity is included in the network to clear the activity pattern in the next two situations

To prepare activities with common starting and ending points noticeable
To discover and maintain the correct precedence relationship between activities that is not linked by events.

For instance, take a situation where A and B are concurrent activities. C is reliant on A and D is reliant on A and B both. This type of situation can be handled with the use of dummy activity as shown in the figure.

2. Event

An event signifies a point in time representing the completion of several activities and the starting of new ones. This is typically symbolizes by a circle in a network which is also known as node or connector.

The events are divided into three categories

Merge event - Event in which more than one activity comes and connects an event is called as merge event.
Burst event - It is event in which more than one activity leaves an event.
Merge and Burst event - An activity can be merge and burst event simultaneously, as regard some activities it can be a merge event and with some other activities it may be a burst event.

3. Sequencing

The first requirement in the development of network is to keep the precedence relationships. In order to prepare a network, the subsequent points must be taken into considerations

What job or jobs follow it?
What job or jobs could run concurrently?
What job or jobs precede it?
What controls the start and finish of a job?

As all further calculations are on the basis of network, it is essential that a network be drawn with full care.

Rules for Drawing Network Diagram

Rule 1

Each of the activity is symbolized by one and only one arrow in the network

Rule 2

No two activities can be recognized by the same end events

Rule 3

To assure the correct precedence relationship in the arrow diagram, various questions must be tested every time any activity is added to the network

What activity must be finished immediately before this activity can start?
What activities must occur at the same time with this activity?
What activities must follow this activity?

In case of big network, it is necessary that certain good habits be applied to draw an easy to follow network

Strive to avoid arrows which cross each other
Make use of straight arrows
Do not try to represent duration of activity through its arrow length
Employ arrows from left to right. Avoid mixing two directions, vertical and standing arrows can be utilized if essential.
Exercise dummies freely in rough draft but final network should be free of any redundant dummies.
The network should have only one entry point known as start event and one point of emergence which is known as the end event.

Common Errors in Drawing Networks

The three types of errors are most frequently seen in drawing network diagrams

1. Dangling

To cut off an activity prior to the completion of all activities in a network diagram is referred to as dangling. As reveal in the figure activities (5 - 10) and (6 - 7) are not the final activities in the network. So the diagram is incorrect and shows the error of dangling

2. Looping or Cycling

Looping error is also called as cycling error in a network diagram. Making an endless loop in a network is called as error of looping as revealed in the following figure.

3. Redundancy

Needlessly put the dummy activity in network logic is called as the error of redundancy as reveal in the diagram below

Advantages and Disadvantages

PERT/CPM has the subsequent advantages

A PERT/CPM chart clearly defines and makes visible dependencies between the elements,

PERT/CPM makes easy identification of the critical path and makes this noticeable,

PERT/CPM makes easy identification of late start, early start, and slack for each activity,

PERT/CPM gives for potentially reduced project period because of better understanding of dependencies leading to enhanced overlapping of activities and tasks where feasible.

PERT/CPM has the subsequent disadvantages:

There can be possibly hundreds or thousands of activities and individual dependency relationships,

The network charts like to be large and unwieldy requiring numerous pages to print and need special size paper,

The lack of a timeframe on the majority PERT/CPM charts makes it difficult to demonstrate status though colours can help (e.g., specific colour for completed nodes),

When the PERT/CPM charts turn out to be bulky, they are no longer utilized to administer the project.