The Critical Path Method is a systematic approach to project planning which can provide significant benefits in cost and completion time of many projects. The concepts were first developed at Dupont, and refined by the United States Navy in the development of the Polaris Missile. They can be applied to various projects, from manufacturing and production, to surgeries and conventions. CPM has found extensive use in the construction industry, where cost and time overruns and frequently experienced. The proper use of CPM scheduling techniques can help you avoid such issues on your own projects.

How is CPM Different From What I'm Doing Now?

You are probably already doing at least some basic work scheduling, so how is the Critical Path Method different? A CPM schedule takes information about tasks, task durations, and task dependencies in order to create a logically sound schedule of your work. This process calculates the minimum time required to complete the project, highlights the areas that are time-sensitive and represent potential trouble for the timely completion of the project, and ultimately produces a graphical representation of the project that can be used to communicate this information to other parties. The identification of the Critical Path, those activities which cause the project completion date to change if they are completed early or late, is the defining characteristic of the Critical Path Method.

How does it compare to other planning methods?

To-Do List (Task List)

This is essentially a list of all tasks that need to be completed in order to complete a project. They may be listed in the order in which they are planned to be completed, but otherwise little information about durations or dependencies is included. This type of planning is usually very fast, and can give a rough idea of the intended work sequence. A shortcoming of this type of planning is that it does not cause you to think exhaustively about the project by exploring the dependencies between activities. It's easy to miss a critical task if you are not thinking in terms of what tasks are dependent on one another. Additionally, while time estimates can be included in a To-Do List, there is nothing that ties the durations of the individual tasks together, so your total project duration cannot be calculated.

Work Calendar

This type of planning expands on the To-Do List by incorporating expected work days for each task, and plotting them against a calendar. These types of schedules are commonly used to avoid crew or equipment scheduling conflicts, by illustrating what work is being performed on a given day. Weather delays and production issues can cause the expected work days to change, which requires manually estimating the new work days, and applying them to the calendar.

Critical Path Method

The critical path method starts in the same place as all other planning, creating a list of Tasks to be performed for the project. Rather than try to assign specific work days to a activity, a task duration is estimated, based on experience and industry standards, and the CPM is used to calculate what the earliest date we can start and finish a task is. Once the Tasks have been identified, the Dependencies between tasks have to be explored, and defined as Relationships. This processes of exploring dependencies will frequently uncover Tasks that were not previously identified, and possibly lead to revision of Task descriptions, work Durations, and creation of additional Relationships. This exhaustive examination of the work process is the foundation of a good schedule.

The Tasks and Relationships are then entered into the Critical Path Method software, and the mathematical algorithms that CPM is based on are run on the provided date. This will then produce an earliest project completion date, early and late start and finish dates for each task to achieve this finish date, and a Critical Path, that identifies the work that is most likely to affect the completion date.

Advantages of the Critical Path Method

A major advantage to this type of planning, is that changes to the schedule can be mathematically evaluated (usually by CPM software, though this can be done by hand) to determine what total effect they have on the project. It will calculate not only the changes to the start or finish of the project as a whole, but individual Task start and finish dates will be recalculated, to ensure any dependencies are satisfied before the planned start and finish. This can also cause a new critical path to emerge, which can result in a change of focus for a project to stay on track.

While the Critical Path Method does require some additional work and research to ensure a thorough schedule is produced, the result is a schedule that remains a useful tool from start to finish of the project, even if significant changes are required.