Elements of a Network
Drawing the logic of a project — events, activities and dependencies
A bar chart shows timing; a network diagram shows the logic — which task must finish before which can start. It is built from activities (tasks that consume time, drawn as arrows) and events (instants, drawn as numbered nodes), with a dummy (a dashed, zero-time arrow) to carry a dependency. A valid network has one start and end, arrows from low to high numbers, and no loops. Spotting what can run in parallel is how a project is shortened without cutting any work.
Learning objectives
By the end of this lesson, you will be able to — mapped to the course outcomes for Project Management in Interior Design:
Distinguish events, activities and dummies, and read a network diagram.
Apply the rules for constructing and numbering a valid network.
Explain how a network shows dependencies a bar chart cannot, and why that matters.
Why a network — events and activities
A plain bar chart shows when; a network shows the dependencies a schedule must respect. It is built from activities (tasks that consume time, drawn as arrows) running between events (instants, drawn as numbered nodes). Keep the two straight: activities have duration; events simply mark moments.[1]
Timing is not logic
A bar chart (Gantt) shows WHEN each task happens — its timing and duration — and it is the most readable schedule there is. But a plain bar chart does not show the LOGIC: which task must finish before which can start, and what depends on what. A NETWORK DIAGRAM does. It models the project as a connected picture of its DEPENDENCIES, so you can see the true sequence, find the critical path, and know what will slip if something is late. This is why network analysis underlies CPM and PERT. Timing and logic are different things: a schedule must respect the logic, and the network is where the logic lives.[1, 2]
The dummy activity
Sometimes the true logic needs a dummy — a dashed arrow of zero duration and resource. It is not real work; its only job is to carry a dependency or keep the network’s numbering valid. A small idea that trips many students: a dummy carries logic, not work.[1]
Logic without work
Sometimes the true logic of a project cannot be drawn with real activities alone, and you need a DUMMY. A dummy is drawn as a DASHED arrow, and it has ZERO duration and ZERO resource — it is not real work. Its only job is to show a DEPENDENCY, or to keep the network's logic and numbering correct (for instance, so that two activities do not share the same start and end events). It is a small idea that confuses many students, so hold it plainly: a dummy carries LOGIC, not work — it says 'this must wait for that', while itself taking no time and costing nothing. Reading dummies correctly is part of reading a network correctly.[1]
The rules, and running in parallel
A valid network has one start and end, arrows from lower to higher numbers, and no loops (a project cannot depend on its own future). Serial activities wait in a line; parallel ones run at the same time — and spotting parallels is how a project is shortened without cutting any work.[1]
How a network must behave
A network follows RULES so it models the project truthfully. It has ONE start event and ONE end event. Every activity runs from a lower-numbered tail event to a higher-numbered head event, so time flows one way. There are NO LOOPS — a project cannot depend on its own future, so an activity can never lead back to an earlier event (that would be nonsense, and it is a genuine error, not a style choice). Two activities should not share the SAME pair of events (a dummy fixes this). Getting these rules right is what makes the network computable — only a valid network can be analysed for its critical path and float.[1]
At a glance
| Aspect | The fact | The folklore |
|---|---|---|
| A bar chart shows | Timing — when and how long | The full dependency logic |
| A network shows | The LOGIC — what depends on what | Only the timing |
| An activity is | A task that consumes time and resource (an arrow) | An instant in time |
| An event is | An instant — the start or finish of activities (a node) | A task with a duration |
| A dummy activity | Carries LOGIC only — zero time, zero cost (dashed) | Real work that takes time |
| Parallel activities | Run at the same time — how a project is shortened | Must always wait in a line |
Key terms
A model of a project as connected events and activities, showing the logical dependencies a schedule must respect — the basis of CPM and PERT.
A task that consumes time and resources, drawn as an arrow, running from a tail event (start) to a head event (finish).
An instant in time — the start or finish of activities, a milestone — drawn as a numbered node; it has no duration or cost.
A dashed arrow of zero duration and zero resource that carries only a logical dependency, or keeps the network's numbering valid — logic, not work.
One start and one end event, arrows from low to high numbers, no loops, and no two activities sharing the same pair of events — what makes a network valid and computable.
Serial activities happen one after another (each waits for the last); parallel activities are independent and run at the same time — spotting parallels shortens the project.
Study task
Take a small interior job — say, refurbishing one room — and list about eight activities (survey, design, order materials, civil work, wiring, flooring, joinery, painting). For each, decide what must finish before it can start. Now draw the network: events as numbered circles, activities as arrows between them, obeying the rules (one start, one end, arrows low to high, no loops). Mark where you would need a dummy to carry a dependency without adding work. Then circle every pair of activities that could run in parallel rather than one after another — and note how much time that could save. The goal is to see a schedule as a structure of logic, not just a row of bars.
Self-assessment
1. What does a network diagram show that a plain bar chart does not?
2. What is the difference between an activity and an event?
3. What is a dummy activity?
4. Why does spotting parallel activities matter?
Recap
References & further reading
- [1]B. C. Punmia & K. K. Khandelwal, Project Planning and Control with PERT and CPM — elements of network analysis: events, activities, dummies, and the rules for constructing and numbering a network. https://www.laxmipublications.com/
- [2]Jerome D. Wiest & Ferdinand K. Levy, A Management Guide to PERT/CPM, Prentice-Hall of India — building and reading project networks. https://www.phindia.com/
Further reading
- B. C. Punmia & K. K. Khandelwal, Project Planning and Control with PERT and CPM.
- Jerome D. Wiest & Ferdinand K. Levy, A Management Guide to PERT/CPM.
Sources gathered and fact-checked June 2026. Published values vary by source, sample and method — treat as indicative and confirm against the cited standard before structural use.
The author
Amogh N P
Architect, interior designer, and creative polymath. Studio Matrx began in his notebooks — his vision of design made honest, useful, and open to everyone. Its Academy is written and taught in his memory, and free, forever.
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