Critical Path Method

CPM

 

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Introduction

·       Critical path is a sequence of activity between a project’s start and finish that takes the longest time to complete

·       Critical Path Method is a mathematical tool that is used to schedule the project activities   

·       It was developed by Kelly and Walker in 1950

 

Critical Path Method (CPM) Formula

Before we learn how to use the CPM formula, we need to understand some key CPM concepts.

 

·       Earliest start time (ES): This is simply the earliest time that a task can be started in your project. You cannot determine this without first knowing if there are any task dependencies

·       Latest start time (LS): This is the very last minute in which you can start a task before it threatens to delay your project timeline

·       Earliest finish time (EF): The earliest an activity can be completed, based on its duration and its earliest start time

·       Latest finish time (LF): The latest an activity can be completed, based on its duration and its latest start time

·       Float: Also known as slack, float is a term that describes how long you can delay a task before it impacts its task sequence and the project schedule. The tasks on the critical path have zero float because they can’t be delayed

The critical path method formula has two parts; a forward pass and a backward pass.

Forward Pass in CPM

·       Use the CPM diagram and the estimated duration of each activity to determine their earliest start (ES) and earliest finish (EF).

·       The ES of an activity is equal to the EF of its predecessor, and its EF is determined by the CPM formula EF = ES + t (t is the activity duration).

·       The EF of the last activity identifies the expected time required to complete the entire project.

Backward Pass in CPM

·       Begins by assigning the last activity’s earliest finish as its latest finish.

·       Then the CPM formula to find the LS is LS = LF – t (t is the activity duration). For the previous activities, the LF is the smallest of the start times for the activity that immediately follows.

Critical Path Method Steps

Now that you know the key concepts of the critical path method, here’s how to calculate the critical path in 8 steps.

1. Collect Project Activities

Use a work breakdown structure to collect all the project activities that lead to the final deliverable.

2. Identify Task Dependencies

Determine which tasks are dependent on other tasks before they can begin. Use your judgment and your team members’ feedback. Failing to define task dependencies correctly makes the critical path method useless.

3. Create a Critical Path Diagram

A CPM diagram or network diagram, depicts the order of activities.

4. Estimate the Timeline

To use the critical path method, you’ll need to estimate the duration of each task. Use data from past projects and other sources of information such as subject matter experts.

5. Use the Critical Path Formula

The critical path uses an algorithm, also referred to as the CPM formula. That algorithm has two parts, the forward pass and the backward pass. The forward pass is determined by using the earliest start for each activity (ES) and the earliest finish (EF). The ES of an activity equals the EF of the one before it. The EF is calculated by EF = ES + t (the duration of an activity).

The backward pass assigns the last activity’s EF as its latest finish. Then use the CPM formula to find the LS, which is LF – t. For the activities before that, LF is the smallest of the start times for the next activity.

6. Identify the Critical Path

The activities with 0 float make up the critical path. All of these critical path activities are dependent tasks except for the first task in your CPM schedule. All project tasks with positive slack are parallel tasks to the critical path activities.

7. Revise During Execution

Continue to update the critical path diagram as you go through the project execution phase. These critical path analysis steps determine what tasks are critical and which can float, meaning they can be delayed without negatively impacting the project schedule. Now you have the information you need to plan the critical path schedule more accurately and have more of a guarantee you’ll meet your project deadline.

Benefits

·       Useful in project management

·       Mathematically simple

·       Give critical path and float time

·       Gives project documentation

·       Useful in monitoring costs

·       Visual representation

Limitations

·       Specified precedence relationship

·       Time estimates are subjective and depends on the judgement

·       Can be difficult to understand

·       Time needed for tasks is not clear

Video Description

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