The critical path method or CPM is a management tool that helps the project manager recognize where in the project schedule his management effort should be applied. The critical path method recognizes that the activities in the schedule that have zero float are the activities that cannot be delayed without delaying the completion of the project. These activities are called critical activities and should be identified as activities that require close supervision.
By this definition of critical activities, all the other activities must have some float. This means that those activities containing at least some float can be delayed without affecting the completion of the project and can be managed somewhat less closely than the critical path activities.
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The critical path method was developed by the Dupont Corporation in the 1950s at about the same time General Dynamics and the U.S. Navy were developing the program evaluation and review technique, which will be discussed next. The objective of CPM is that it can take projects that have reasonably well known task durations and identify the tasks that should be monitored more closely by management. CPM is therefore a management technique.
The critical path is really somewhat misleading since the critical activities do not necessarily have to fall on a path. The idea of a path comes from a more simple-minded approach to the critical path method where the critical path is determined by finding the longest path through the project network diagram, shown in NETWORK DIAGRAM WITH CRITICAL PATH.
By using this technique you can find all of the possible ways of getting from the start end to the finish end of the network following the various branches of the interdependencies. This will give you every possible sequence of activities from the start of the project to the finish. When all the paths have been determined, you add up the durations for each possible path. When this is done, you declare that the critical path is the one with the highest total duration.
Obviously this will not simply produce the right answer if you have any kind of relationship in the network diagram other than a finish-start relationship or if you have leads and lags.
The proper method and the method that the computer software for project management uses is to determine the total float of each activity and identify those activities that have zero total float. These activities may sometimes not form a path through the network. They may even form two paths through the network.
To manage using the critical path, we use the float in each activity as the criterion for managing. Activities that have little or no float are managed much more closely than activities having more float. Since the activities having little or no float will cause project completion delays if they are late, they are obviously the ones upon which we must concentrate our management effort.
Of course, since this is a management tool and we are using it to determine where management effort should be concentrated, we should also consider the activities that have close to zero float since they are nearly critical. In the project management software, the number of days of free float allowed before an activity becomes critical is adjustable so that we can show activities with, say, three days or less of free float. To make this even easier, the software will highlight these activities by showing them in red (or a color of your choice).