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Critical Path Method Scheduling

Basic Components of a Critical Path Method


Critical Path Method Scheduling

Schedule sample.

Courtesy of jwalker64
Critical Path Method scheduling was developed in 1957, to address challenges on complex projects. Being one of the most used scheduling techniques in the construction industry, the Critical Path Method is a useful tool that can lead you to achieve your project results.

The Critical Path Method schedule shall include all work specified in the Contract Documents, including all expected activities of subcontractors, vendors, suppliers and all other parties associated with construction of the project.

A critical path method will provide you with a graphical view of the project, will provide you the time required to complete an activity and determine the activities that will become critical to the project if not completed within the specified time.

Understanding a Critical Path Method (CPM)

A critical path method (CPM) is a network of events, each one of them linked to the following activities. Each activity is represented as a node on the network, and connecting lines are drawn to represent the time schedule to complete that activity. A critical path method schedule must be completed using the following steps:

  1. Identify the activities
  2. Determine the sequence of the activities
  3. Connect or create a network of the activities
  4. Enter the completion time for every activity listed on step 1
  5. Identify the critical path or the longest possible path to complete all activities
  6. One important and valuable component is the CPM update progress, allowing to track closely the performance and time used to complete the activities

Identify the Activities

A complete list of activities needed to complete the project must be made. This list can be prepared from the work breakdown structure or the project scope and details. This will be the key to later add sequence and duration in subsequent steps. Activities shall be identified by a name, stationing, and coding, and have a duration.

Determine the Sequence of the Activities

This is one key factor of a critical path method (CPM). A complete and thorough understanding of the sequence of the activities are needed to prepare and connect the list of activities prepared in the previous step. Linking activities must be analyzed and connected properly to achieve the expected result, failing to do so can lead to mistakes and miss-representation of the project activities, leading to a false time frame or erroneous project completion date. If one crew is being used to perform similar activities, these activities must be linked together.

Creating the Network

When all activities have been defined and interdependence of activities has been analyzed, it is required to create the network of activities. It is of vital importance that all critical work sequence has been linked with logical coordination and planning requirements. The CPM format shall be based on calendar days as their main planning unit. Working days should be avoided when planning or preparing a critical path method schedule.

Estimate Activity Completion Time

Using previous experience, time estimates can be presented, and shall represent the necessary time to complete the activity for a single resource unit. It is one of the most important steps when preparing a CPM, because it will help you determine the time needed or available to perform an activity.

Identify the Critical Path

The critical path is the longest-duration path through the network. What does this mean? Activities located on this path cannot be delayed without delaying the project. Because of its impact on the entire project, critical path analysis is an important aspect of project planning. The critical path can be identified using these parameters:
  • ES – Early Start: earliest time to start a predetermined activity, given that prior activities must be completed first
  • EF – Early Finish: earliest finish time for the activity
  • LF - Late Finish: latest time the activity must be completed without delaying the entire project
  • LS – Late Start: latest start date that the activity must be started without delaying the project
  • The critical path is the path through the project network in which none of the activities have been delayed, that is, the path for which ES=LS and EF=LF for all activities in the path. A delay in the critical path delays the project.

    Update CPM Diagram

    As the project is being developed, a critical path method can be updated. Analyzing a new critical path may be possible, when entering all the duration times of activities already completed. A new path might be found and alternative solutions can be presented to either accelerate the project or continue working as projected. Sometimes it is also necessary to incorporate changes and extra works that were not part of the original critical path.
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