Example of a PERT

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Critical Path Method (Copyright © Arch Media Group LLC)

The critical path method, abbreviated as CPM, is the most commonly used type of scheduling technique in the construction industry. The critical path method is built around the use of predecessors and successors for each construction task needed to accomplish the job. Most tasks (except the initial task) will have one or more predecessors that need to be accomplished before that task can start. If we select a certain task, such a framing a house, we need to determine what must occur before the framing can be constructed. In this case the ground floor slab will need to be finished before the framing can be placed on top of it. Therefore, creation of a ground floor slab is the predecessor of the house framing task. Conversely, framing the house is a successor of the ground floor slab construction.

Following the placement of tasks in the schedule, along with the creation of linkages between the predecessor and successor tasks, a critical path will become apparent. The critical path is formed by the longest chain, time wise, of interrelated tasks from the project start to the project finish. Basically, if any of the tasks in the longest chain were to take longer than their planned duration, the length of the project would be extended. The extension happens because the schedule slippage in one component in the critical path will push back the start dates of that component’s successor tasks. Therefore, the project manager places a high priority on these critical tasks, in order to keep the project on schedule.

Tasks that are located off of the critical path will often contain what is known as float. Float is an important concept, and is basically spare time contained within tasks that are located outside of the critical path. For example, the installation of a building roof may be scheduled for four weeks but in reality it is only expected to take three weeks. The extra week is float contained within that task, and the actual duration of work may “float” within that time period, sliding backwards or forward as needed to accomplish the project goals. If a project task that contains float happens to run over schedule, it can use the float time to finish and not affect the completion of the overall project.