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How Can You Use the Critical Path Method?

written by: chemuturi • edited by: Ronda Bowen • updated: 7/22/2013

This article explains the features of the critical path method (CPM), namely, network diagram, forward pass, backward pass, critical activities, slack and crashing.

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    What Is It?

    What are the Features of Critical Path Method? The Critical Path Method (CPM) is a network diagram-based project planning and monitoring method. A network diagram depicts the activities of the project pictorially. All the activities are embedded and connected together between the start event and the end event of the project.

    CPM is used together with PERT (Program Evaluation and Review Technique) as PERT/CPM as both are network diagram-based project planning and monitoring methods. While PERT originated in the research and development environment, CPM evolved in the construction industry.

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    Features of CPM

    The Critical Path Method helps us to identify the critical activities of the project and thereby the duration of the project. Start by estimating project duration. We carry out the forward-pass and the backward-pass to compute the project duration. In forward-pass we begin at the start event of the project and add the duration needed by each activity to the date on which the start event would begin. Thus, we reach the end event. Now the date on which the end event is reached is the expected project completion date.

    In backward-pass we begin at the end event of the project subtracting the duration needed by each activity from the project completion date progressively till we reach the start event. The date of the start event arrived at this manner is the date the project should begin in order to deliver the project on time.

    Now, each activity on the network has two sets of dates – one arrived at through forward-pass and the other through backward-pass. The dates of activities arrived through forward-pass are known as “earliest times" – earliest start and earliest finish – for each activity. The dates of activities arrived through backward-pass are known as the “latest times: - latest start and latest finish – for each activity. The earliest start date denotes the date on which the activity can be started and the latest start denotes the date on which the activity must start in order not to delay the project. Similarly, the earliest finish date of an activity denotes the earliest date by which the activity can be finished and the latest date denotes the date by which the activity must finish in order not to delay the project.

    The difference between latest and earliest (earliest & latest start or earliest & latest finish) dates of an activity is called the “slack" for that activity. The activities for which the slack is zero are referred to as the “critical activities." The path of activities from the start event to the end event is called the “critical path" for the project when the slack of all activities on that path is zero. The sum of durations of the activities on the critical path is the duration of the project.

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    Crashing

    The CPM assumes that by applying extra resources, the project duration can be reduced. Crashing is the systematic exercise of reducing the duration of the project at the least possible increase in cost in order to complete the project earlier than expected. Crashing is an iterative exercise needing multiple iterations.

    CPM is a deterministic model – that is the duration for the activities is known in advance with confidence. Each activity would have two durations, namely, the “normal duration" and the “crash duration". Associated with the normal duration is the “normal cost" and the cost associated with crash duration is the “crash cost". The relationship between the normal and crash values is assumed to be linear. We compute the cost-time slope (extra cost per day of reduction in duration) for each activity on the critical path.

    Then we select the activity that has the least cost-time slope and crash (reduce its duration) it. Then we re-compute the new critical path. We also see if the objective (the duration required for completing the project) is reached. If it is achieved, then we do not iterate further. If the objective is not achieved, we iterate again till the objective is achieved.

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    Summing It Up...

    Summarizing the above discussion, these are the features of CPM –

    1. Visualizes the project activities using a network diagram
    2. Gives the duration and the completion date for the project
    3. Identifies the critical and non-critical activities of the project so that we can focus more on the critical activities and complete the project on time.
    4. Allows us to complete the project earlier than completed using the technique of crashing.

    Image Credit: Image courtesy of sxc.hu/ohinsanity