Birth of the Critical Path Method

CPM is the most dominant scheduling network analysis technique, first developed by the company E I du Pont de Nemours (DuPont) through research in 1956, which led to the influential work of James Kelley and Morgan Walker in 1957, who used the term CPM “because of the central position that critical activities in a project play in the method”. One of CPM’s prime success stories is its use in the Apollo 11 Project, which landed two men on the moon on July 20, 1969. NASA used the CPM to help determine an efficient schedule for 2 million tasks that led to the moon landing.

To establish a meaningful Critical Path, it is necessary to develop a logic-based network of activities with empirically derived durations for execution in a realistic and practical manner. The project duration is computed by applying the algorithm of Forward Pass and Backward Pass, which compute the ‘earliest’ and ‘latest’ start/finish times for every activity.

The early start and finish times are the earliest time that each activity can be started and finished, given the technical precedence relationships in the network. The latest start and finish times are the latest an activity can be started and finished, without delaying the total time to complete the project.

The difference between the early and late start (finish) time is the ‘Total Float’ or ‘Total Slack’ associated with an activity. The activities with zero total float cannot be delayed without delaying the entire project. These activities are on the longest path through the network, which is called the Critical Path.

To form the schedule network, the work is divided into a set of interdependent activities grouped under a work breakdown structure. The relationships between individual activities are defined in the network by using the Arrow Diagramming Method, Activity on Arrow, Precedence Diagramming Method or Activity on Node.

Scheduling using CPM has established its capacity to successfully manage time in complex projects. By 1962 it had demonstrated time reductions against manual methods of approximately 25 percent, as well as representing a significant cost saving.

Strengths

• Maximises efficiency in the use of time
• Makes dependencies between activities visible
• Provides a systematic approach for scheduling projects
• Calculates the total float for each activity
• Supports project compression by providing capability to analyse and crash the critical path
• Supports delay analysis and claim/dispute resolution
• Creates a platform for impact analysis of the identified risks and opportunities

Limitations

• Relies on the deterministic estimation of activity duration
• Accuracy is dependent on having reliable data
• A sophisticated software tool is required
• Only provides one set of the deterministic critical path
• Its efficiency may be limited in projects with repetitive processes
• Creates less focus on non-critical activities, which can ultimately cause risks and delays