Theory of Constraints Handbook - James Cox Iii [47]
In CC project plans, it is vital to resolve all resource contention with reverse passes through the project schedule; that is, starting from the end of the project schedule, eliminating resource contention all the way back to the start of the project. Following this resource leveling effort, the Critical Chain is identified as the longest chain of task and resource dependencies. Ideally, a Critical Chain remains the same throughout project execution.
Merging Paths
There is special risk in a project schedule where paths or chains5 of dependent activities merge with other chains. If one of the paths is the Critical Chain, the project completion date can be endangered by late completion of a non-critical path. As we will see in a sample CC project schedule, special attention is ascribed to chains of dependent activities that merge into tasks on the Critical Chain.
Communications
There are many policy differences between traditional project management and CCPM and those differences will require changes in organizational and individual behaviors. An especially important process for CC projects is an effective communication system that includes a method of resource notifications, a message to a resource to: (1) start a chain (path) of activities, (2) prepare for upcoming work on the Critical Chain, or (3) perform critical work on a higher-priority project in a multi-project environment. Such notifications help to ensure that CC tasks, which determine project completion, will be given appropriate priority.
Later in the chapter, we will describe how CC overcomes all the forces that pose challenges to successful project completion.
Issues in Managing Project Execution
Ideally, no project should be started unless all specifications have been received, the charter has been approved, an acceptable schedule has been approved, and all other preparatory steps have been accomplished. Further, no task should be started unless all required materials are available and the task is at the start of a FIFO work queue. Having everything ready and on hand before starting a project or a task is referred to as having a “whole kit” or a “full kit.” While a research project may violate this “rule,” other projects should not.
In traditional project management, once a project is begun, each task is managed as if it were an independent event. That is, a worker is rewarded if an assigned task is completed on or before its scheduled finish date; exhorted to work harder if it is not completed on the finish date; and punished, in several ways, if the finish date is overrun by a significant amount. The rationale for this partitioning of project work is that if every task is completed on time, the project will be completed on time. Of course, this rationale completely disregards the reality that few, if any, tasks are passed on early. Therefore, if only a few tasks complete late, as nearly always happens, the entire project is delayed.
Critical Chain uses buffers to manage task duration uncertainty and to monitor project progress. A later section, entitled “Project Control: The Power of Buffer Management,” describes how this is accomplished.
Scheduling a Single Project
One of the easiest ways to illustrate the way CC addresses the issues presented previously is by contrasting what is done in traditional project environments with a single project example. To illustrate the scheduling steps, we use a simple project with 5 resources and 10 activities (tasks). There is only one of each of the five uniquely qualified resources; each resource can perform its own work but cannot perform the work of any other resource.
To provide a better understanding of single-project scheduling, a manual CC process is described in the following section. However, there are software programs now available that can perform these scheduling steps in both single project and multi-project environments.