The Critical Path Method was designed in the 1950s as a way to manage projects in the Polaris missile defense program. While CPM can be a useful project management tool for determining how to effectively get from a project start to a successful completion, it is not always the best method. Understanding the strengths and weaknesses of CPM is vital for successful project management.
CPM organizes projects as a series of interconnected activities, much like a tree diagram or a flow chart with multiple paths between the beginning and end. By analyzing the project using this model, the project manager can see all of the different paths that can be taken and then identify the shortest or most effective path, which is the critical path.
A major limitation of CPM is that, being activity-based, it does not allow you to track people or resources independently of these activities. If a person is reallocated during part of the project, or if one person is required to work on overlapping activities, the CPM in itself can't reflect this. A CPM chart could falsely lead you to believe that a single person or resource can be at two places at the same time.
Even planning a small project can result in a complicated CPM chart. For large projects with many activities, creating a CPM analysis, and then trying to manage a project with it, can be exceedingly complex. Large projects often have more than one activity that needs to be worked on simultaneously. Because the major benefit of CPM is identifying the shortest path to project completion, it is not always suitable for projects with a lot of overlapping activities.
If CPM is to be used effectively, you need accurate estimates for the time it will take for each activity to be finished. If you are unable to accurately estimate these times, trying to identify the critical path becomes a guessing game. Even a small project can be hampered by using CPM without accurate time estimates. The less experience you have with a project, the less likely it is that CPM will be useful in managing the project.