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
- The event at the start of the project is called the source node
- 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
- For each activity, the table includes a list of the activities that must already have been completed
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
