Critical Path Method (CPM) for Construction Management

The critical path method is a type of network analysis used in construction and project management. The CPM is a project schedule method by dividing the construction procedure into a set of activities and representing them in a network diagram. Each activity represented in a CPM network represents the duration required to complete that particular activity. Critical Path Method (CPM) is also called Critical Path Scheduling.

As per Project Management Body of Knowledge (PMBOK),

"  The Critical Path Method is the sequence of scheduled activities that determines the duration of the project."

CPM Network Diagram
Image Credits: Smart Learn

In the above figure, A is an activity with start and end nodes and path which  (1 and 2), the duration of an activity A is 3, given the start and finish times of the activity in the column. 

In CPM method, among various working path in the network, the objective of project managers is to determine the critical path-which is the path that have critical activities.

The CPM method was developed in the late 1950s by Morgan R Walker and James E. Kelly. The method originated in common with the Program Evaluation and Review Technique (PERT) method. Both CPM and PERT methods are used in combination depending on the complexity of the project.

Also Read: Objective and Applications of PERT and CPM Project Scheduling Techniques

Objectives of the CPM Method

The CPM network technique is commonly used by project managers to accurately plan construction projects. They are used to:

1. Accurately estimate the total project duration.
2. To identify the task dependencies, the resources constraints, and the possible project risks.
3. To identity critical tasks, prioritize tasks and create realistic project schedules.

Elements of Critical Path Method

In the CPM method, it is essential to determine the time duration of each activity and its interdependency with other activities, in order to derive the CPM network method. The critical path analysis is performed based on this. The time parameters studied on each the  activity in CPM method are:

1. Earliest Start Time (ES): This is the earliest start time at which the activity can be started. This can only be dependent on knowing task dependencies activity.
2. Latest Start Time (LS): This is the very last minute or latest time at which the project activity can be started without delaying the whole project schedule.
3. Earliest Finish Time (EF): This is the earliest time an activity can be completed, based on the duration and ES of the activity.
4. Latest Finish Time (LF): The latest finish time is the last minute or latest time the activity must be completed without affecting the overall project duration. It is based on the LS and the activity duration.
5. Float/Slack: Float defines a duration or value that describes how long an activity can be delayed before it impacts its task sequence and the project schedule. Float can be Total Float or Free Float.
1. Total Float: It is the time that any activity can be delayed from the early start without delaying the finish date or deviating the schedule constraint. Total Float TF= LS -ES or LF -EF.
2. Free Float: This defines how long an activity can be delayed without impacting the following activity (succeeding activity). Free float is possible when two or more activities share a common successor. Free Float FF= ES ( next task) - EF ( current task)

The elements are represented on an activity node as shown in below figure. It an CPM network, each activity is represented as shown.

Critical Path and Critical Activities

A critical path in a CPM network is the path with activities having float = 0. The activities in the critical path are called critical activities.

Critical activities are activities that can no way be delayed and any delay in their completion affects the whole project duration. 

The main features of the critical path in the CPM network are:

1. The critical path is the longest sequence of tasks in the project. 
2. The total time taken to complete the critical path sequence gives the minimum project duration.
3. Any delay in the activities of the critical path affects the whole project.
4. The float of activities of a critical path is equal to zero.

Critical Path Highlighted in Black Bold

Working of Critical Path Method (CPM)

Working of CPM is the steps and procedures involved in determining the critical path and analysis of the critical path. The basic steps involved in determining the critical path are:

1. Listing the project tasks and details
2. Identify the task dependencies for the project
3. Create a network diagram of the tasks
4. Estimate each task duration
5. Find the critical path based on the float and longest sequence
6. Calculate the total float

Also Read:

1. How to find the Critical Path and Duration of a Construction Project? 
2. Characteristics of PERT or CPM
3. Forward and Backward Pass- Determine Start and Finish Time of an Activity