Learn About Critical Path Method Scheduling

Critical Path Method is one of the most used scheduling techniques in the construction industry due to its simplicity and powerful resource that it can be. The Critical Path Method is a useful tool that can lead you to achieve your project results and help you delivering the project on time. 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 CPM generates a graphical view of the project and is used to calculate how much time and resources are required to complete an activity.  It also determines critical activities requiring attention so the project can be completed on 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 ni 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

    You might want to start developing a list of activities for the project, normally that information is obtained 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 are identified by name, coding, accounting string or other and must have a duration or target date in the case of milestones.

    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. The project or construction manager needs to understand the activities that are linked or are related so they can show the proper connection in the schedule. Sometimes contractors fail to identify these relationships causing delays and other problems during project execution.

    Creating the Network

    The CPM is created summarizing all defined activities and showing interdependence between them. 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. A CPM schedule should avoid using working days as it might lead to confusion and errors.

    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.

    How to 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.

    How to 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 possible 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.