activity-networks-and-precedence-tables

Activity networks & precedence tables

What is an activity network?

• An activity network is a graph that shows the activities needed – and in what order – to complete a project

  • e.g. the project could be ‘building a house’ with activities such as ‘foundations’, ‘walls’ and ‘roof’

• Some activities will depend on others being completed first

  • e.g. the activity ‘foundations’ would need to be completed before the ‘walls’ are built

• Some activities can occur at the same time

  • e.g. ‘windows’ and ‘doors’ can be fitted at the same time

• The arcs (edges) of the graph represent the activities

  • this may be referred to as an activity-on-arc network

• The nodes (vertices) of the graph represent events within the project

  • events can be thought of as ‘stepping stones’

  • the project cannot progress beyond an event until all the activities leading to that event are completed

    • ‘you can’t jump off the stepping stone until all activities leading to that stone are completed’

What does an activity network look like?

• Events (Nodes) are labelled with numbers, generally increasing in the direction of the project

  • The event at the start of the project is called the source node

    • it is labelled with 0 or S

  • The event at the end of the project is called the sink node

    • it will be the highest numbered node or labelled with T

• Arcs are labelled with their activities, with the duration given in brackets

  • activities are denoted by capital letters – A, B, C, D, etc

  • arrows are drawn on the arcs to show the order in which the project progresses

    • so strictly speaking, an activity network is a directed graph

    • in broad terms, this is generally from left to right across the activity network

What is a precedence table?

• A precedence table shows a list of the activities for a project

  • For each activity, the table includes a list of the activities that must already have been completed

    • but only the immediately preceding activities are listed, not all of them

  • Activities that do not have any precedents are indicated by ‘-‘

    • these activities can begin at the start of the project

    • they will be attached to the source node

What does a precedence table look like?

• As well as a list of activities a precedence table may also show

  • the duration of each activity

  • the number of workers required to complete that activity

Drawing an activity network

How do I draw an activity network?

• An activity network can be drawn from a precedence table

• Starting with the source node

  • add an arc for each activity, one at a time, considering its immediately preceding activities

  • an event (node) will be needed prior to each activity commencing

    • more than one activity can commence from the same event

    • more than one activity can finish at the same event

• A crucial feature of an activity network is that each activity has a unique pair of start and end nodes

• Any activities that do not precede another will go to the sink node at the end of the project

• In general, activity networks

  • use straight, arrowed lines for arcs

  • numbered circles for events/nodes