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These rules are used to compute the backward pass of the wind energy example. First the LF of activity J (50 workdays as mentioned above) is subtracted with its duration (LF – Dur = LS; 50 – 5 = 45). The thereby calculated LS of J is directly carried to activity I where it becomes its LF. The LS of activity I (45 – 5 = 40) is again directly transferred to activity H (LF). The LS of activity H (40 – 10 = 30) may directly affect the two activities G and B. In the case of activity G it is clear that the LS of activity H is directly transferred to the LF of G, because activity H is its only immediate following activity. The LF of activity B is controlled by the LS of activity H. The latest activity B can be finish is 30 days. The LFs of activities C, D, E and F are only depending on activity G so their LF becomes 25. The LS dates of activities B, C, D, E and F which are all affecting the LF of the first activity A are computed below.
- LS(B) = 20 – 10 = 20 - LS(C) = 25 – 10 = 15 - LS(D) = 25 – 20 = 5 - LS(E) = 25 – 15 = 10 - LS(F) = 25 – 10 = 15
As with the rules detailed above, the smallest LS of activity B, C, D, E, and F is the right choice for the LF of activity A, in this case the LS of D, because activity D takes the longest time to complete in this comparison. The LS of activity A (5 – 5 = 0) completes this backward pass, all latest activity times are known.
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Figure 4-9 shows the totally filled in graph after the calculations are made:
Figure 4‑9: Finished Graph
4.4.3 Identification of Slack or Float
When the forward and backward passes have been computed, it is possible to determine which activities can be delayed by computing “slack” or “float”. Total slack or float for an activity is simply the difference between the LS and ES (LS – ES = SL) or between LF and EF (LF – EF = SL). Figure 2-11 shows different examples, slack for activity B is 15 days, for activity E 5 days and for I 0 days. The slack gives information about the amount of time an activity can be delayed without delaying the whole project. If slack of one activity in a path is used, the ES for all activities that follow in the chain will be delayed and their slack reduced. Use of total slack must be coordinated with all participants in the activities that follow the chain. After slack for each activity is computed, the critical path(s) is (are) easily identified. When the LF = EF for the end project activity, the critical path can be identified as those activities that have LF = EF or a slack of zero (LF – EF=0 or LS – ES = 0).
The critical path in the suspension bridge example is represented by activities A, D, G, H, I and J (Figure 4-10). A delay in one or more of these activities would delay the whole project.
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Figure 4‑10: Critical Path
A network schedule that has only one critical path and non-critical activities and that enjoys significant slack would be labeled insensitive. Conversely, a sensitive network would be one or more critical paths and/or non-critical activities with very little slack. Under these circumstances the original critical path is much more likely to change once work gets under way on the project.
AON-networks have different advantages. For example, they are easy to draw and the logical structure and the simple elements used help the project manager to design such networks. Another advantage is that indirectly involved persons, like first-level managers, are able to understand the main points and problems of a project quite quickly by using an AON-Network because the graphical representation helps to bring everything that influences the project into well arranged context .
The computations needed to develop an AON-Network are quite simple and easy to handle. On the other hand, an AON-Network without a graphical printout is virtually useless, because only a table including all facts and figures with a graphical representation can be read efficiently? AON-Networks created with modern computer software can help the project manager to arrange his daily work in an efficient way and to concentrate on other important tasks.
60 4.4.4 Scheduling Techniques
Network scheduling techniques form the basis for all planning and predicting and help management decide how best to use its resources to achieve time and cost goals. Managers can cope with the complexities, masses of data and tight deadlines that are characteristic of highly competitive industries by using these techniques.
They make all steps of a project more transparent, so it’s easier to recognize dependencies between activities, to schedule risks, to identify critical paths and to evaluate how delays will influence project completion.
There are several different scheduling techniques, but the most common ones are network diagrams like the AON method and the Program Evaluation and Review Technique (PERT).1
The Program Evaluation and Review Technique is a network analysis technique which uses the AOA or AON approach to estimate project duration. PERT has the ability to deal with uncertainty in activity completion times. It can help to develop more realistic schedules to reduce cost and time requirements.
This is a great advantage compared to the critical path method. The CPM is more deterministic and uses fixed time estimates for each activity. Time variations, that can have a great impact on the completion time of a complex project, will not be considered. For the performing of PERT estimates, a three-point estimate for each activity is required. A three-point estimate is an activity duration estimate that includes an optimistic, most likely and pessimistic estimate. The optimistic estimate is based on a best-case scenario.
Generally, it is the shortest time in which the activity can be completed. The most likely estimate is based on an expected scenario. The completion time has the highest probability. The pessimistic estimate is based on a worst-case scenario. That is the longest time that an activity might require.
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