4.2 Developing the critical chain solution
4.2.1 Identifying the project constraint
Defining the constraint of a project in terms of the schedule derives from the impact that schedule has on project cost and project scope. Inde- pendent variables that influence a project result include the demanded scope, the project system definition, and the resources available to work on the project. The project system outputs are dependent variables (deliv- ered scope, cost, and schedule). As schedule increases with fixed deliver- able scope, cost usually increases. As scope increases with fixed cost (or resources), schedule tends to increase. As scope increases with fixed schedule, cost tends to increase. Therefore, it is appropriate to focus first on delivering the project on time.
The evident constraint of a project is the chain of tasks that takes the longest to complete. To perform any task on a project, two things are nec- essary: the task input from a predecessor and the resource to perform the task. (The predecessor may simply be a start authorization for the first task in a chain of project tasks.) The definition of the critical path does not explicitly address the potential resource constraint. Section 3.3.2 described why the critical chain is the single-project constraint. The criti- cal chain is simply “the sequence of dependent events that prevents the project from completing in a shorter interval. Resource dependencies determine the critical chain as much as do task dependencies.”
Figure 4.2 illustrates a typical critical path project schedule. The letters represent unique resources. For this illustration, assume there is only one
resource corresponding to each letter, that is, only one A resource, only one B resource, and so on. Evidently, we would fail to meet the schedule on the project, because each resource can do only one task at a time.
Figure 4.3 moves tasks to eliminate the overlap of resource demands.
In a manner similar to many computer algorithms for resource leveling, we first give the resource to the path with least float; usually with the ini- tial critical path. Note that when we are done, all paths have float, so there is no critical path defined as the path with zero float. (Computer software packages treat that result differently. Some keep the initial critical path definition. Some define only the last task as critical. I have no idea what they do about the critical path as the project progresses and the critical path is supposed to change.)
More importantly, the initial critical path is not the constraint to completing the project. Because the resource constraint is often a signifi- cant project constraint, the TOC method of project planning always considers it (Figure 4.4).
If your organization does not have resource constraints (or has infinite resources), the critical chain will be the same initial task path as the critical path. That is an important fact in verifying the integrity of the critical chain method; it contains the critical path method as a special case, at least in regard to defining the critical chain.
A B C
E C
B
A C D
Critical path
Resource
Time Figure 4.2 The critical path does not account for the resource constraint.
The PMBOKGuide definition of critical path states that the critical path may change during the performance of the project. That occurs when project tasks experience common cause variation and redefine the longest zero float path to complete the project. Based on our knowledge of variation, that means we should expect the apparent critical path to change frequently. Dr. Deming noted that one of the more serious mis- takes managers can make is to treat common cause variation as if it were special cause variation. That PMBOKGuide definition of critical path (and implementation in many project management systems) institu- tionalizes that mistake. It does not enable the project team to focus on the constraint to the project but causes them to make the error of chasing an ever changing critical path. As Dr. Deming illustrated with the funnel experiment, that always makes the project system perform- ance worse.
A B C
E C
B
A C D
Resource leveled critical (?) path
Time Figure 4.3 Removing resource conflicts usually creates gaps in the critical path.
A B C
C D
Figure 4.4 The critical chain includes both the resource and task logic constraint to completing the project on time or sooner.
The critical chain does not change during project performance, partly a matter of definition but mostly a result of the overall critical chain plan construction procedure and the subordination step.