Fundamentals Of Project Management

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Unformatted text preview: . Fundamentals of Project Management by James P. Lewis AMACOM Books ISBN: 0814478352 Pub Date: 01/01/95 Search Tips Search this book: Advanced Search Previous Table of Contents Next Title Backward Pass Computations ----------- A backward pass is made through the network to compute the latest start and latest finish times for each activity in the network. (See Figure 6-3.) To do this, we must decide how late the project can finish. By convention, we generally don’t want a project to end any later than the earliest possible completion; to stretch it out longer would be inefficient. We also won’t insist (for now) that our yard project end earlier than the earliest possible finish calculated in the preceding section. If we want to finish earlier we will have to redraw the network or shorten some activities (by applying more resources or working more efficiently, for example). For now, we will accept the 165-minute working time and let it be the Latest Finish for the project. Figure 6-3 CPM diagram for yard project (filled in). If Hauling Trash has a Late Finish of 165 minutes and has a duration of 45 minutes, what is the latest that it could start? Subtracting 45 from 165 leaves 120 minutes, which is the Latest Start for the task. Proceeding in this manner, we get LS times for Bagging Grass and Bundling Clippings of 90 and 105 minutes, respectively. One of these two numbers must be the LF time for each of the preceding activities. Which one? When doing backward pass calculations, always use the smallest number for the LF of previous activities. Well, assume we try 105 minutes. If we do that, Bagging Grass could start as late as 105 minutes, since as soon as earlier tasks are finished, subsequent tasks can begin. But if we add 30 minutes for Bagging to the 105-minute ES time, we will finish at 135 minutes, which is later than the 120 minutes previously determined, and we will miss the 165-minute end time for the project. When you are doing backward-pass calculations, the Latest Finish for a preceding task will always be the smallest of the Late Start times for the subsequent tasks. (A simpler way to say this is: Always use the smallest number!) Rule: When two or more activities follow a third activity, the latest time that the earlier activity can be achieved is the smaller of the times. All of the activities that have ES/LS or EF/LF times that differ are said to have float. For example, Trim Weeds has an ES time of fifteen minutes and a LS time of sixty minutes, giving it forty-five minutes of float. When an activity has no float, it is called critical, since failure to complete work as scheduled will cause the end date to slip. Now examine the path that includes activities highlighted by bold lines. Each activity has the same ES/LS and EF/LF times. There is no float on this path. By convention, an activity with no float is called critical, and a total path with no float is called a critical path; if any of the work on...
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