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Some Characteristics of the Critical PathSome Characteristics of the Critical PathWe have so far described the critical path as the longest path through the network. This is true and it is one of the clearest and most defining characteristics of the critical path. A second idea is that there is no float or slack along the critical path. Having no float or slack means that if there is any change in durations along the critical path, then the overall schedule will be longer or shorter. In effect, such a characteristic means there is no schedule "reserve" that can isolate vagaries of the project with the fixed business milestones. In point of fact, almost no project is planned in this manner and the project manager usually plans a reserve task of time but not performance. We will see in our discussion of the "critical chain" concept that this reserve task is called a "buffer" and is managed solely by the project manager. Figure 7-3 illustrates this idea. For purposes of identifying and calculating the critical path, we will ignore the reserve task. A third idea is that there can be more than one critical path through the network. A change in duration on any one of the critical paths will change the project completion date, ignoring any reserve task. Another notion is that of the "near-critical path." A near-critical path(s) is one or more paths that are not critical at the outset of the project, but could become critical. A path could become critical because the probabilistic outcomes of durations on one of these paths become longer than the identified critical path. Another possibility is that the critical path becomes shorter due to improved performance of its tasks and one of the "near-critical" paths is "promoted" to being the critical path. Such a set of events happens often in projects. Many project software tools have the capability of not only identifying and reporting on the near-critical path, but also calculating the probability that the path will become critical. Moreover, it is often possible to set a threshold so that the project manager sees only those paths on the report that exceed a set probability of becoming critical. In addition, it is possible to identify new paths that come onto the report or old paths that drop off the report because of ongoing performance during the course of the project. Lastly, if there is only one connected path through the network, then there is only one critical path and that path is it; correspondingly, if the project is planned in such a way that no single path connects all the way through, then there is no critical path. As curious as the latter may seem, a network without a connecting path all the way through is a common occurrence in project planning. Why? It is a matter of having dependencies that are not defined in the network. Undefined dependencies are ghost dependencies. An early set of tasks does not connect to or drive a later set of tasks. The later set of tasks begins on the basis of a trigger from outside the project, or a trigger is not defined in the early tasks. Thus the latter tasks appear to begin at a milestone for which there is no dependency on the earlier tasks. In reality, such a network is really two projects and it should be handled as such. If addressed as two projects, then each will have a critical path. The overall length of the program (multiple projects) will depend on the two projects individually and the ghost task that connects one to the other. Such a situation is shown in Figure 7-4. 2972
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