Project Management (CPM/PERT): role of dummy activities in a network diagram In network planning (arrow diagrams), why are dummy activities introduced into the CPM/PERT network?

Introduction / Context:
In critical path method (CPM) and program evaluation and review technique (PERT), network logic must correctly represent all precedence relationships among activities. Sometimes two or more activities share head or tail events that would otherwise create ambiguity. To resolve this without inventing non-existent work, planners use a special construct known as a dummy activity.

Given Data / Assumptions:

• Arrow-on-activity representation is being used.
• Each pair of nodes must uniquely identify an activity.
• Precedence relationships can be complex and non-series-parallel.

Concept / Approach:
A dummy activity has zero duration and zero resource requirement. It carries no work. Its only purpose is to maintain correct logic by showing that a certain activity cannot start until one specific predecessor (or set of predecessors) is finished, even when other activities share the same nodes. This preserves one-activity-per-node-pair and prevents parallel activities from being confused as dependent when they are not.

Step-by-Step Solution:
Identify the network ambiguity: two real activities would otherwise occupy the same tail and head events.Insert a dummy (dashed arrow, duration = 0) to split or reorder relationships.Recompute earliest and latest times; the dummy does not change durations or float, only logic.Hence dummy activities are used to maintain correct network logic and unique precedence, not to compute time directly.

Verification / Alternative check:
If the same problem is modeled in an activity-on-node diagram, no dummy is needed because precedence can be drawn directly. This confirms that dummies are purely a modeling device for arrow-on-activity networks.

Why Other Options Are Wrong:
Option (A) and (B): critical path and project duration are outputs of time analysis; dummies do not determine them. Option (E): dummies never replace real tasks. Option (D) is invalid.

Common Pitfalls:
Assigning duration or resources to dummies; using dummies to “speed up” a schedule; forgetting one-activity-per-node-pair rule.

Final Answer:
to maintain the required network logic and unique precedence relationships