---
title: "Dummy activity; use of critical path in CPM"
subject: "Project Engineering"
chapter: 3
key: "semester_7/project_engineering:dummy-critical-path-use"
source: https://www.bctengineer.com/q/semester_7/project_engineering/dummy-critical-path-use
site: BCT Engineering
---

# Dummy activity; use of critical path in CPM

Project Engineering · Chapter 3: Project Planning and Scheduling

- Asked in: 2076 Chaitra, 1+3 marks
- Video: [Dummy activity for Critical Path Method](https://www.youtube.com/watch?v=6P1AfgiVwWk) — Fuad Abutaha
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/project_engineering/dummy-critical-path-use
- Whole chapter: https://www.bctengineer.com/md/semester_7/project_engineering/chapter-3

## Question

What is a dummy activity? Write down the use of critical path in a CPM network diagram.

## Short answer

A dummy is a zero-duration, zero-resource activity drawn as a dashed arrow to show logical dependence or give unique event numbers. The critical path in CPM gives the minimum project duration, identifies zero-float critical activities needing priority control, guides crashing, and allows float on non-critical activities for resource levelling.

## Answer

#### Dummy activity

A **dummy activity** is an activity of zero duration and no resource, drawn as a dashed arrow, used in an arrow diagram to show logical dependence or to give unique event numbers.

#### Use of critical path in a CPM network

- Gives the **minimum time** to complete the project (the length of the longest path).
- Identifies the **critical activities** that have no float; they need close control and priority in resources.
- Shows that a delay of a critical activity delays the whole project, so delay is predicted and corrected early.
- To shorten the project duration (crashing), only the critical path is shortened.
- Non-critical activities have float, which is used for **resource levelling** and flexible scheduling.
- Helps in cost-time trade-off and in updating the schedule during execution.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is a dummy activity used for?
   A. Consuming resources
   B. Showing logical dependence or giving unique event numbers
   C. Adding duration
   D. Marking milestones
   Answer: B. It has zero duration and no resource and is drawn dashed. (Key point: Dummy)

2. What does the critical path give?
   A. The minimum time to complete the project
   B. The maximum cost
   C. The total float of the project
   D. The average of all paths
   Answer: A. The longest path length is the minimum project duration. (Key point: Minimum time)

3. In crashing, which path is shortened?
   A. Any non-critical path
   B. The shortest path
   C. The critical path
   D. Every path equally
   Answer: C. Only shortening the critical path reduces the duration. (Key point: Crashing)

4. How is float on non-critical activities used?
   A. To lengthen the critical path
   B. To remove dummies
   C. To increase cost
   D. For resource levelling and flexible scheduling
   Answer: D. Float lets activities be shifted without delaying completion. (Key point: Non-critical use)

## Related in Project Engineering

- [Critical path and dummy activity significance](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Acritical-path-dummy-significance) — Project Engineering, chapter 3
- [CPM network, 9 activities (A=3 to I=1)](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Acpm-network-nine-act-b82) — Project Engineering, chapter 3, asked 5×
- [CPM network, 9 activities (A=5 to I=2)](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Acpm-network-nine-act-a75) — Project Engineering, chapter 3, asked 5×
- [CPM network, 13 activities (A to M)](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Acpm-network-thirteen-act-k82) — Project Engineering, chapter 3, asked 5×
- [CPM network, 13 activities (A=2 to M=7)](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Acpm-network-thirteen-act-b81) — Project Engineering, chapter 3, asked 5×
- [Dummy activity and its application](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Adummy-activity-application) — Project Engineering, chapter 3

## Related in other subjects

- [CPM schedule, critical path and float](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_engineering_and_management%3Acpm-network-critical-path-float) — Industrial Engineering and Management, chapter 3
- [TCTO numerical: crashing critical path, Rs 30/day](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atcto-2069-bhadra) — Construction Management, chapter 2
- [Essential components of an activity diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Aactivity-diagram-components) — Object Oriented Analysis and Design, chapter 2