Critical Path Method Scheduling

The critical path method (CPM) is a popular scheduling technique in the construction industry due to its simplicity and effectiveness. It generates a graphical view of a project and calculates how much time and resources are required to complete each activity. It also determines critical activities requiring attention so that the project can be completed on time.

A CPM lists all activities in a project and includes information about how long each activity will take and how each relates to the completion of other activities.

A Basic CPM Example

Consider a simple example of a project with three activities:

• Activity A will take three months.
• Activity B also will take three months, but cannot begin until Activity A is complete.
• Activity C will take two months and is independent of both Activity A and Activity B.

From these basic details, we know that the fastest the project can be completed is six months because Activities A and B each will take half that time, and B cannot be started until A is complete. To meet that six-month timeframe, it is critical that Activity A is started first so that Activity B can be started immediately afterward. Activity C is less critical and can be completed during any two-month window during the six-month timeframe.

This, of course, is a very stripped-down example. Most projects are more complex, with more overlapping and interconnected activities. Because of all of those moving parts, a CPM can improve efficiency at achieving project results while delivering them on time, and should include all work specified in the contract documents, including all expected activities of subcontractors, vendors, suppliers, and other parties associated with the construction of the project.

Building a CPM takes six basic steps.

1. Identify the Activities

Start by developing a list of activities for the project, normally obtained from the work breakdown structure or the project scope and details. Sequence and duration will be added in subsequent steps. Activities are tagged by name, coding, accounting string, or other identifier and must have a duration or target date in the case of milestones.

2. Determine the Activities' Sequence

A complete and thorough understanding of the sequence of the activities is 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 related so that they can show the proper connections in the schedule. Contractors sometimes fail to identify these relationships, causing delays and other problems during project execution.

3. Create the Network

The CPM evolves from summarizing all defined activities and showing the interdependencies between them. All critical work sequences need to be linked with logical coordination and planning. The CPM format should be based on calendar days as a main planning unit. A CPM schedule should avoid using working days, as such an approach is more likely to lead to confusion and errors.

4. Estimate Activity Completion Time

Using previous experience, project time estimates for how long it will take to complete activities for a single resource unit.

5. Identify the Critical Path

The critical path is the longest duration path through the network. 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 path for each activity 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.

6. Update the CPM Diagram as Needed

As a project is being developed, you can update the critical path. 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 also is necessary to incorporate changes and extra tasks that were not part of the original critical path.