What does a node represent?

An event, the start or finish of activities.

What is an immediate predecessor?

An activity that must be finished immediately before another can start.

What does a precedence table show?

Each activity with its immediate predecessors only.

What is a dummy activity?

A zero-duration arc that shows a dependency but has no work or resources.

When is a dummy needed for logic?

When two activities share some, but not all, of their predecessors.

Second use of a dummy?

To give two activities with the same start and end events different end events.

How do you find an activity's predecessors from a network?

Look at the activities ending at its start event, including those reached through dummies.

Are dummies listed in a precedence table?

No; trace back through them to the real activities.

Activity networks and precedence tablesEdexcel A-Level Further Maths: Flashcards

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What does an arc represent in an activity-on-arc network?

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What does an arc represent in an activity-on-arc network?: An activity.
What does a node represent?: An event, the start or finish of activities.
What is an immediate predecessor?: An activity that must be finished immediately before another can start.
What does a precedence table show?: Each activity with its immediate predecessors only.
If C depends on B and B depends on A, what does C's row show?: Only B.
What is a dummy activity?: A zero-duration arc that shows a dependency but has no work or resources.
When is a dummy needed for logic?: When two activities share some, but not all, of their predecessors.
Second use of a dummy?: To give two activities with the same start and end events different end events.
How are dummies drawn?: As dotted arrows.
How do you find an activity's predecessors from a network?: Look at the activities ending at its start event, including those reached through dummies.
Are dummies listed in a precedence table?: No; trace back through them to the real activities.
How many start events does the network have?: One.

Exam questions on Activity networks and precedence tables

A project has activities P, Q, R, S and T. The immediate predecessors are: P none; Q none; R is P; S is P and Q; T is R and S. The project is to be modelled by an activity network with the activities on the arcs.
Explain why a dummy activity is needed in this network.2 marks
A project is modelled by an activity network with events numbered 1 to 6. The activities, with their start and end events, are: A (1 to 2), B (1 to 3), C (2 to 4), D (3 to 4), E (4 to 5), F (3 to 5) and G (5 to 6). There is also a dummy activity from event 2 to event 3.
State the immediate predecessors of activity E and of activity F.2 marks
A project has activities A, B, C, D and E. The immediate predecessors are: A none; B none; C is A and B; D is B; E is C and D. The project is to be modelled by an activity network with the activities on the arcs.
Explain why one dummy activity is needed, and state where it must be placed.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).