Project Scheduling

Network Based Scheduling

This technique is used for bigger and complex projects involving large number of activities. In this technique network diagrams are constructed.

Network Diagram: it is a graphical flow plan of activities arranged in a logical sequence that must be accomplished for completing a project. There are two popular network scheduling techniques:

  1. Critical Path Method (CPM)
  2. Programme Evaluation and Review Technique (PERT)

Advantage of Network Technique

  • Precedence relationships
  • large projects
  • more efficient

How to construct Network Diagram:

Activity: All projects are composed of various operations or tasks, which require time and resources to complete. These tasks and operations are called an activity. For example in the previous illustration digging of foundation, pouring foundation concrete and construction of walls are the activities of a project constructing a shopping mall.

An activity is represented by an arrow 

  1. The length of the arrow does not signify anything.
  2. Each activity has a significant beginning and a definite end.
  3. Each activity consumes resources (time, manpower and material).
  4. The head of the arrow shows the flow of activity.
  5. The circle placed at the beginning of the arrow represents the starting point of the activity, while the circle placed at the end of the arrow represents the finishing point of the activity.
network based scheduling

Predecessor Activities: Activities that must be completed immediately before the start of another activity are called predecessor activities of it.

network based scheduling

Project Time Cost Trade-off Project Time Cost Trade-off 

Activity A is the predecessor of Activity B

Successor Activities: Activities that must follow immediately after completion of a given activity are called successor activities of it.

network based scheduling

Activity B is the successor of activity A.

Concurrent Activities: Activities that can be completed at the same time are called Concurrent Activities.

network based scheduling

A and B are concurrent activities.

Dummy Activity: A dummy activity is an imaginary activity added in a network to maintain the logical sequence only. Since it is not a real activity, it does not consume any resources (time, manpower and materials). It is represented by a dashed arrow.

network based scheduling

Example 1: Draw the network diagram for the following project::

  1. Activity A and activity B are concurrent activities.
  2. Activity C follows activity A.
  3. Activity D succeeds activities A and B
  4. Activities C and D are last activities.
network based scheduling

Event: An event is the beginning and end of an activity. An event represents a specific point in time. It does not consume time, manpower or material resources. In the network diagram an event is represented by a circle.

network based scheduling

Network Rules:

  1. Each activity is represented by one and only one arrow in the network. Therefore no single activity can be represented twice in the network.
  2. No two activities can be identified by the same beginning and end events. In such cases a dummy activity is introduced to resolve the problem.

3. Before an activity can be undertaken, all activities preceding it must be completed.

4. The arrows depicting various activities are indicative of the logical precedence only. The length of the arrow has no significance.

5. The flow of diagram should be from left to right.

6. Arrows should not be crossed.

7. Arrows should be kept straight and not curved/bent.

8.The general rule for numbering the event is that no event can be numbered until all preceding events have been numbered. The number at the head of an arrow is always larger than at its tail.

  1. Identify the initial event and assign it number 1.
  2. Delete all the emerging arrows from the initial event (event 1). This will create one or more ‘new initial events’, number these initial events as 2, 3, 4……….etc.
network based scheduling

3. Delete all the emerging arrows from the initial events which will create a new set of initial events. Assign numbers to these initial events starting from the number next to the number that has so far been assigned.

Example 2: Number the events in the following network:

network based scheduling

network based scheduling network based scheduling 

Interdependency

  • If activity A is the predecessor of activity B, it means activity B can not start until activity A is completed.
Project Scheduling network based scheduling
  • If activity A is the predecessor of activities B and C both, it means activities B and C can not start until activity A is completed.
network based scheduling
  • If activities A and B are the predecessors of activity C, it means that activities A and B can occur concurrently but both must be completed before C can begin.
network based scheduling
  • Activities A and B both must be completed before activities C and D can begin independently.
network based scheduling
  • Activities A and B can occur concurrently but both must be completed before activity C can begin. However only activity B must be completed before activity E can begin. Activity C is a dummy activity which shows a precedence relationship but has zero time duration.
network based scheduling

network based scheduling network based scheduling 

network based scheduling
Project Scheduling network based scheduling
Project Scheduling network based scheduling

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By Hassham

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