問33
頻出度★★★
システム開発プロジェクトにおけるクリティカルパスに関する記述のうち, 適切なものはどれか。
平成23年度 特別試験 ITパスポート試験 問33
マネジメント系プロジェクトマネジメント › プロジェクトマネジメント
This question's explanation and your answers so far.
解説
問 33
Incorrectア
It reverses where staff should be added first, so it is wrong. To recover a delay, that is, to shorten the overall duration of the project, staff must be added precisely to the tasks on the critical path, because those tasks are what determine the overall duration. Tasks that are not on the critical path already have float, so no matter how much earlier they are finished, the length of the longest path does not shrink by even one day and the overall completion date does not change (limited staff would simply be allocated where they produce no effect). Note also that as the critical path is shortened, another path becomes the longest and turns into the new critical path, so after each shortening the paths must be recalculated and the next focus target reviewed.
Incorrectイ
Asserting that it necessarily happens is what makes this wrong. If a task on the critical path is completed 3 days ahead of schedule, that path's duration does become 3 days shorter, but if as a result another path that had been the second longest becomes longer than it, the overall project duration is then determined by that new longest path. For example, when the longest path is 40 days and the next longest is 38 days, shortening the longest path by 3 days brings it only to 37 days, and the 38-day path becomes the new critical path, so the whole project moves up by 2 days rather than 3. The effect of shortening is at most 3 days and may in practice be less; it is not necessarily a 3-day advance. Note that this is not symmetric with the case of a delay (choice ウ), where the path grows longer, no switch of the longest path occurs, and the whole project is always affected.
- ウ
正解 The correct answer is ウ: If a task on the critical path is delayed, the project completion is also delayed. The critical path is the longest path, the one whose total duration in days is the largest, among all the paths that can be traced from start to finish when tasks are connected by their predecessor-successor order in an arrow diagram (PERT chart). Since the project is complete only when every task has finished, the overall duration of the project is determined by the length of this longest path itself. In other words, tasks on the critical path have no float at all (float being the difference between the latest node time and the earliest node time), so even a delay of a single day is added directly to the length of the longest path, with no slack to absorb it, and pushes back the project completion date. For example, in a project that takes 40 days overall, if a task on the critical path is delayed by 2 days, completion moves in principle to day 42. This is exactly why progress management monitors the tasks on the critical path closely and acts on them first at the first sign of delay. Tasks on paths other than the critical path, by contrast, have float, and a delay within that float does not affect the overall completion. This asymmetry is the very essence of the concept of the critical path, so ウ is appropriate.
Incorrectエ
There is not necessarily only one, so this is wrong. The critical path is defined as the path whose total duration in days is the largest, and there is no rule that this maximum can appear on only one path. If two or more paths share the same largest total duration, all of them are critical paths, and several exist at the same time. In that case a delay in a task on any one of those paths delays the whole project, so the number of tasks that must be monitored closely increases accordingly. Furthermore, as tasks on the critical path are shortened, the duration may end up tied with another path so that there are several critical paths, or the critical path may shift to a different path.
ここがポイント
- The critical path is the longest path, the one whose total duration in days is the largest, among all the paths that can be traced from start to finish on an arrow diagram (PERT chart), and it is the path whose float is zero. The overall duration of the project is determined by the length of this path, so a delay in a task on the critical path becomes, directly, a delay in the overall completion. Conversely, a task that is not on the critical path does not affect the whole project as long as its delay stays within its float.
- Delay and shortening are not symmetric. A delay makes the longest path even longer, so it always propagates to the whole project, whereas during shortening another path may take over as the longest, so the days saved do not necessarily translate into the same advance for the whole project. Therefore, staff added to recover a delay should go first to tasks on the critical path, and after each shortening the paths should be recalculated to confirm the new critical path. When durations end up tied, several critical paths can exist at the same time.
用語