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Unformatted text preview: assumption that unlimited resources are available. This yields
the best-case solution. BASIC SCHEDULING COMPUTATIONS
Scheduling computations in this section are based on the network in Figure 6-1. First, let us examine the node
boxes in the schedule in Figure 6-2. Each contains the notations ES, LS, EF, LF, and DU.
ES = Early Start
LS = Late Start
EF = Early Finish
LF = Late Finish
DU = Duration (of the task) Figure 6-1 CPM diagram for yard project. Forward Pass Computations
Consider a single activity in the network, such as picking up trash from the yard. It has a duration of fifteen
minutes. Assuming that it starts at time = zero, it can finish as early as fifteen minutes later. We therefore can
enter fifteen in the cell labeled EF.
Putting gas in the mower and the weed whacker takes only five minutes. The logic of the diagram says that
both of these tasks must be completed before we can begin trimming weeds, cutting the front grass, and
edging the sidewalk. The cleanup task takes fifteen minutes, whereas the gas activity takes only five minutes.
How soon can the following activities start? Not until the cleanup has been finished, since it is the longest of
the preceding activities. Figure 6-2 Forward pass computations for yard schedule.
In fact, then, the Early Finish for cleanup becomes the Early Start for the next three tasks. It will always be
true that the latest Early Finish for the earlier tasks becomes the Early Start for subsequent tasks. That is, the
longest path determines how early subsequent tasks can start.
The Earliest Start for a task is the latest Late Finish of preceding tasks. That is, the longest path determines
the earliest that a subsequent task can be started.
www.erpvn.net Following this rule, we can fill in Earliest Start times for each task, as shown in Figure 6-2. This diagram
shows that the project will take a total of 165 minutes to complete, if all work is conducted exactly as shown.
We have just performed what are called forward pass computations to determine Earliest Finish times for all
activities. Computer programs do exactly the same thing and also convert the times to calendar dates, making
quick work of the computations.
Rule: When two or more activities precede another activity, the earliest time that the final activity can
be started is the larger of the durations of the activities preceding it.
The time determined for the end or final event is the earliest finish for the project in working time. Once
weekends, holidays, and other breaks in the schedule are accounted for, the end date may be considerably
later than the earliest finish in working time. Previous Table of Contents Next Products | Contact Us | About Us | Privacy | Ad Info | Home
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This document was uploaded on 09/27/2013.
- Fall '13