---
title: "Difference between CPM and PERT"
subject: "Industrial Engineering and Management"
chapter: 3
key: "semester_6/industrial_engineering_and_management:difference-between-cpm-and-pert"
source: https://www.bctengineer.com/q/semester_6/industrial_engineering_and_management/difference-between-cpm-and-pert
site: BCT Engineering
---

# Difference between CPM and PERT

Industrial Engineering and Management · Chapter 3: Loading and Scheduling Technique

- Asked in: year not given, 5 marks
- Video: [PERT Vs CPM: Difference between them with definition & Comparison Chart](https://www.youtube.com/watch?v=dFTG3ohAcso) — Key Differences
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_engineering_and_management/difference-between-cpm-and-pert
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_engineering_and_management/chapter-3

## Question

Differentiate between CPM and PERT. Explain the terms critical path, float and event slack.

## Short answer

CPM uses deterministic time estimates for repetitive projects, while PERT uses three probabilistic estimates for uncertain R&D projects. Critical path is the longest path determining project duration. Float is delay allowed without delaying project; event slack is difference between latest and earliest event times in PERT.

## Answer

Both **CPM** (Critical Path Method, DuPont 1957) and **PERT** (Program Evaluation and Review Technique, US Navy, Polaris project 1958) are network techniques for planning, scheduling and controlling projects.

| Basis | CPM | PERT |
|---|---|---|
| Origin | Industrial/construction projects | Research and development projects |
| Time estimate | One deterministic time | Three estimates: a, m, b |
| Nature | Deterministic | Probabilistic |
| Orientation | Activity oriented | Event oriented |
| Cost | Time-cost trade-off (crashing) is considered | Mainly time; cost not considered |
| Use | Repetitive, well-known jobs | Non-repetitive, uncertain jobs |
| Probability | Not computed | Probability of meeting a date is found |
| Distribution | Not used | Beta distribution |

#### Terms
- **Critical path:** the longest path through the network; it fixes the shortest possible project duration. Any delay in a critical activity delays the whole project.
- **Total float:** the time by which an activity can be delayed without delaying the project completion: $TF = LS - ES = LF - EF$. Critical activities have zero float.
- **Free float:** delay possible without affecting the earliest start of the next activity.
- **Event slack (PERT):** for an event, $\text{slack} = T_L - T_E$, the difference between latest allowable and earliest expected event times. Events on the critical path have zero slack.

## Check yourself

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

1. How do the time estimates differ?
   A. CPM uses three, PERT uses one
   B. CPM uses one deterministic time, PERT uses a, m and b
   C. Both use three
   D. Neither uses time
   Answer: B. CPM is deterministic; PERT is probabilistic with three estimates. (Key point: Time estimates)

2. Which technique considers time-cost trade-off (crashing)?
   A. PERT
   B. Neither
   C. Both equally
   D. CPM
   Answer: D. CPM considers time-cost trade-off; PERT is mainly time. (Key point: Crashing)

3. What does total float mean?
   A. Time by which an activity can be delayed without delaying project completion
   B. Time between two events
   C. Time of the shortest path
   D. Time by which the project can be crashed
   Answer: A. TF = LS - ES = LF - EF; critical activities have zero float. (Key point: Total float)

4. What is event slack in PERT?
   A. TE minus TL
   B. TL - TE, latest allowable minus earliest expected event time
   C. Sum of activity times
   D. Always 1
   Answer: B. Events on the critical path have zero slack. (Key point: Event slack)

## Related in Industrial Engineering and Management

- [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

## Related in other subjects

- [Difference between CPM and PERT](https://www.bctengineer.com/question.md?key=semester_7%2Fict_project_management%3Acpm-vs-pert) — ICT Project Management, chapter 8
- [Difference between CPM and PERT](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Acpm-vs-pert-difference) — Project Engineering, chapter 3, asked 2×
- [Dummy activity; use of critical path in CPM](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Adummy-critical-path-use) — Project Engineering, chapter 3
- [Bar chart vs CPM/PERT as planning tool](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Aplanning-tool-choice) — Construction Management, chapter 2, asked 3×