Project Planning: Critical Path and Critical Chain
Among project management tools, a project schedule is the plan component that lists all work to be completed for on-time project delivery. The schedule shows project tasks to be done along with required organizational resources and allocations. Timing information such as milestones, deliverables, and start/finish dates is usually included. Nowadays, online project management software is used to handle scheduling and many other aspects of project planning. The designs of these digital project planning tools are based on theory and practice embodied in two traditional approaches to project planning: critical path method (CPM) and critical chain project management (CCPM). Here is a brief introduction to these classic project planning paradigms.

Critical Path Method

CPM was developed in the late 1950s by engineers at DuPont and Remington Rand. The method was based on earlier strategies conceived at DuPont between 1940 and 1943 and put into practice during the Manhattan Project. CPM can be used on any type of project that involves interdependent activities.

The basic CPM approach involves creating a list of activities and tasks needed to finish a project, estimating the time needed to complete each task, identifying dependencies between activities, and setting one or more logical endpoints. With this information in hand, planners calculate the longest path in terms of activities that must be completed to reach the designated endpoint, and the earliest each task can start and latest it can finish without extending total project duration. Activities that fall on the longest path are called critical activities, and their durations determine overall project duration. Thus, the critical path is the sequence of tasks that adds up to the longest project duration. Identifying the critical path is a way to determine the shortest time possible for project completion.

Non-critical tasks are activities that can be delayed without extending project duration. Float is the name for extra time that becomes available because of constraints or dependencies that affect the start times of particular tasks. In some cases, it may be a resource that can be used for project work, but it does not add to project completion time. Non-critical tasks are said to have “total float”.

Critical Chain Project Management

CCPM is a more recent (1997) model for project planning and management that was developed by Eliyahu M. Goldratt, the Israeli business management expert and author who originated the Theory of Constraints. The CCPM method is based on the management of required resources rather than task order and rigid scheduling. Specifically, the method is dependent on the use of resource leveling, an effort to balance the demand for resources with the available supply by adjusting task start and finish dates to account for resource constraints.

The planning process in CCPM is similar to that used in CPM. The plan begins with a completion date and is worked backward from there. Each task is assigned two durations, one that assumes about a 50% probability of on-time completion and another that has a higher probability. Once resources are assigned to the tasks and the plan is resource leveled using the shorter durations, the critical chain is formed by the longest sequence of resource-leveled tasks running from beginning to end of the project. The critical chain is the sequence of precedence- and resource-dependent tasks that determines the shortest possible time for project completion.

In an environment of unlimited resources, the critical chain and critical path of a project are identical. In CCPM, the difference between the low and high probability completion times is totaled and used as buffer, with project delivery set for the end of the buffer period. Therefore, buffer is time specifically allotted to allow for statistical variation from original duration estimates.


There has been much debate in the project management literature about the benefits of CPM vs. CCPM. However, it can be seen that both methods share certain aspects. CPM and CCPM offer different perspectives on a variety of issues in project planning, and astute project managers will take advantage of techniques from both approaches.

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