Theory of Constraints Project Management

TOC Project Management: Project Scheduling
Upon completion of the network building process, the resulting network is used to determine the project schedule with a process commonly called Critical Chain Scheduling.

Task, resource, and iteration dependencies are analyzed to determine the longest “chain of work,” called the Critical Chain. The process then separates what can be thought of as the “fixed” component of the work from the “variable” component of the work. The fixed component of each task (and each iteration sequence) is the aggressive but possible estimate. The variable component (also called “safety”) is the difference between the aggressive but possible estimate and the corresponding highly probable estimate. This safety is then removed from each task location and aggregated with the safety from other tasks in the same chain of work. A portion of the aggregated safety is placed in strategic locations where it will serve to protect the project as a whole. These placements are called buffers.

A Project Buffer, located between the end of the Critical Chain and the project’s commitment date, protects the project from the effects of execution variability along the Critical Chain. Feeding Buffers, located every place a non-Critical Chain task feeds a Critical Chain task, protect the Critical Chain from execution variability along the paths that feed it.

Figure 2. A simple project to illustrate the placement of time buffers.

The mathematical properties of aggregation, combined with new ways of working (see below) on project tasks, lead to a reduction in the total amount of safety that is required – much less than traditional methods would require to protect each task separately. This leads to shorter overall project duration and, in some cases, reduced project costs.

The resulting Critical Chain schedule is both feasible and immunized. Feasible, in that resource contention is properly accounted for and immunized, in that safety is placed in strategic locations where it will protect the project as a whole. The insertion of buffers also staggers path starts, leading to clearer priorities for resources, a reduction in the tendency for significant bad multitasking, and a reduced potential for mistakes.