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CRITICAL PATH METHOD (CPM)

Dr Adewale Abimbola, FHEA, GMICE.

www.edulibrary.co.uk

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Aim: Critical Path Method (CPM)

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Objectives: At the end of the lesson, the students should be able to:

  • Define critical path method.
  • Discuss the significance of critical path in sequencing of construction operations.
  • Produce a critical path analysis of a construction project.
  • Discuss the similarities and differences between the Program Evaluation & Review Technique (PERT) and CPM.

Aim and Objectives

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Introduction

  • In general, two construction planning methods are adopted in practice by most contracting organisations. These are bar charts and networks.
  • Networks are usually adopted by the organisation for the purpose of providing highly detailed programming for a project. They draw attention to the activities on the programme which are on the critical path. This makes them very good for monitoring and controlling the progress of a project.
  • Examples of network analysis diagrams are Program Evaluation and Review Technique (PERT) and Critical Path Method (CPM).
  • Bar charts used for their clarity and ease of communication in conveying the basis of the programme schedule.

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Network Analysis

  • Networks are often presented as one of two types: as ‘arrow diagrams’ or as ‘precedence diagrams’ (Figs. 1 & 2).
  • The two methods essentially achieve the same goal, but with different means of depicting the information.
  • In this lesson, only the Critical Path Method (CPM) will be considered.

Figure 1. Nomenclature for compiling the Program Evaluation and Review Technique (March, 2017).

Figure 2. Nomenclature for compiling the Critical Path Method (March, 2017).

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The Critical Path Method (CPM) - Definition

  • CPM is employed by project managers for deadline determination in project modelling.
  • Understanding activity relationships is crucial for creating a network diagram and program schedule.
  • The critical path represents the longest route from project start to finish, encompassing all tasks and durations.
  • Identifying the critical path provides a clear view of the project's real-time schedule.

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The Critical Path Method (CPM) - Definition

  • Automated tools simplify critical path calculations, eliminating the manual process.
  • Project scheduling software automates critical path calculations, simplifying the entire process.
  • Understanding the Critical Path Method and its automated calculations is integral for effective project management and timely project delivery.
  • Let's analyse an example to enhance comprehension.

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The Critical Path Method (CPM)

  • Here some concepts you need to understand:
  • ES - Early Start - is the earliest the task can start.
  • EF - Early Finish - is the earliest the activity can finish.
  • LS - Late Start - is the latest the activity can start without delaying the project (affecting the Critical Path).
  • LF - Late Finish – LF is the latest the activity can finish without delaying the project (affecting the Critical path).

Figure 2. Nomenclature for compiling the Critical Path Method (March, 2017).

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Worked Example

Using the information in the table below;

  1. Construct the project network or logic.
  2. Determine the expected project completion time.
  3. Find the critical path.

Activity/Task

Duration

(days)

predecessor

A

3

​

B

8

A

C

10

A

D

12

C

E

15

B

F

9

D, E

G

13

F

H

5

F

I

2

A

J

5

G, H, I

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Worked Example: Solution

1) Construct the project network or logic.

Activity/Task

Duration

(days)

predecessor

A

3

​

B

8

A

C

10

A

D

12

C

E

15

B

F

9

D, E

G

13

F

H

5

F

I

2

A

J

5

G, H, I

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Worked Example: Solution

2) Find the expected project completion time.

Complete the forward pass to determine the project completion time of 53 days.

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Worked Example: Solution

3) Find the critical path.

Complete the backward pass to confirm the project completion time and determine critical path.

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Dependency Relationships – CPM

  • Finish to Start (FS):
    • Definition: The dependent task cannot start until the preceding task is completed.
    • Example: Wall construction (dependent task) starts only after the foundation is finished (preceding task).
  • Finish to Finish (FF):
    • Definition: The dependent task cannot finish until the preceding task is completed.
    • Example: The inspection of completed floors (dependent task) finishes when the entire construction project inspection is concluded (preceding task).
  • Start to Start (SS):
    • Definition: The dependent task cannot start until the preceding task starts.
    • Example: Excavation for utilities (dependent task) starts concurrently with the initial site clearing (preceding task).
  • Start to Finish (SF):
    • Definition: The dependent task cannot finish until the preceding task starts.
    • Example: Training sessions (dependent task) may finish when the project starts (preceding task) to ensure a knowledgeable team from the project outset.

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Benefits – CPM

  • Provides a clear and visual representation of the project schedule.
  • Facilitates efficient allocation of resources.
  • Ensures logical sequencing of project tasks.
  • Critical path allows you to sharpens focus on meeting project deadlines.
  • Enhances communication and coordination among project teams.
  • Establishes a baseline for progress monitoring.

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Limitations– CPM

  • Complex network diagrams for large projects.
  • CPM assumes that activity durations are fixed. In reality, activity durations can be uncertain or subject to change, affecting the accuracy of the critical path.
  • Relies on a single time estimate for each activity. This simplicity may overlook the inherent uncertainty and variability in task durations.
  • External factors, such as changes in market conditions, are challenging to integrate.
  • Maintaining and updating detailed project documentation can be resource-intensive, especially in fast-paced environments.

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Self-assessment Task 1 – CPM

Using the information in the table below;

  1. Construct the project network or logic.
  2. Calculate the expected project completion time.
  3. Find the critical path.

Task

Activity

Days ending

Predecessor

A

Strip site - topsoil to a depth of 150 mm minimum

3

​

B

Excavate to reduced level

4

A

C

Set out house foundations

2

B

D

Excavate foundation trenches (including ducts for services)

3

C

E

Cast concrete to deep strip foundations

5

D

F

Brickwork to dpc (damp proof course) level

5

E

G

Fill cavity below dpc level

2

F

H

Supply and compact hardcore to ground floor slab (including 50 mm sand blinding)

6

F,G

I

Lay dpm (damp proof membrane)

2

H

J

Spread and compact hardcore to ground floor slab

5

I

K

Excavate trenches for drainage/inspection chambers & lay them

10

I, J

L

Test drain for leaks and backfill drainage

4

K

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Self-assessment Task 2 – CPM

Using the information in the table below;

  1. Construct the project network or logic.
  2. Calculate the expected project completion time.
  3. Find the critical path.

Activity/Task

Duration (weeks)

predecessor

A

1

-

B

3

-

C

9

A

D

7

B, G

E

4

C, D

F

2

D

G

6

A

H

3

E

I

4

-

J

1

I

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References/Bibliography

  • March, C. (2017) Construction management – theory and practice. London: Routledge.
  • Morton, R. (2002) Construction UK: Introduction to the industry. Oxford: Blackwell Science.
  • Zornes, T. (n.d.) Critical path method (cpm). Available at: https://sixsigmastudyguide.com/critical-path-method-cpm/ (Accessed: 07 November 2023).

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