The critical path method (CPM) is a technique for scheduling a series of project activities. Under CPM, each task is arranged in sequential order, along with a time estimate for how long it will take to complete the task. An event occurs when a task either begins or is completed. This information is then displayed on a CPM chart. This chart is useful for determining how delays will influence the completion of a project, where there is slack in a project, and which tasks are crucial for meeting the project due date.
Slack time occurs when there are activities that can be completed before the time when they are actually needed. The difference between the scheduled completion date and the required date to meet the critical path is the amount of slack time available. The project manager should always be aware of where slack time exists in a project, since this time can be used to reshuffle the schedule to support the critical path. For example, if there is slack time in a task not located on the critical path, resources can be shifted from that task to tasks located on the critical path, thereby bolstering the most crucial tasks. One can also keep track of the trend in available slack time for each task. If the trend is declining, it can indicate that work is taking longer than expected.
Of particular interest in the chart is when tasks that could hold up the project are identified as critical. By arranging the chart to focus attention on the critical tasks, one can forecast the time required to complete the entire project. The critical path is the longest path through a network. There is no slack in any task on the critical path, so if there is a delay in any of these tasks, the completion of the entire project will be delayed. Consequently, a project manager’s attention is heavily focused on ensuring that each task along the critical path begins and ends on time.
It is possible that there are several near-critical paths associated with a project. If the tasks within these paths become prolonged and their float is consumed, it is entirely possible that one of these paths will become the new critical path, supplanting the original critical path. If this happens, the attention of the project manager is then focused on the new critical path. When there is a near-critical path, it behooves the project manager to watch it closely over time and evaluate the risk that it may become the new critical path.