Using dummy activities

When two paths within a network have a common event although they are, in other respects independent, a logical error such as that illustrated in Figure 6.13 might occur.

Activity Network Diagram Dummy

Suppose that, in a particular project, it is necessary to specify a certain piece of hardware before placing an order for it and before coding the software. Before coding the software it is also necessary to specify the appropriate data structures, although clearly we do not need to wait for this to be done before the hardware is ordered.

Figure 6.13 is an attempt to model the situation described above, although it is incorrect in that it requires both hardware specification and data structure design to be completed before either an order may be placed or software coding may commence.

We can resolve this problem by separating the two (more or less) independent paths and introducing a dummy activity to link the completion of data structure design to the start of the activity placing an order. This effectively breaks the link between data structure design and placing the order and is shown in Figure 6.14.

Design data c°de software >

Figure 6.14 Two paths linked by a dummy activity.

Dummy activities, shown as dotted lines an the network diagram, have a zero duration and use no resources. They are often used to aid in the layout of network drawings as in Figure 6.15. The use of a dummy activity where two activities share the same start and end nodes makes it easier to distinguish the activity end-points.

Figure 6.15 Another use of a dummy activity.

Exercise 6.2

Where parallel activities have a time lag we may show this as a ladder' of activities. In this case documentation may proceed alongside prototype testing so long as it starts at least a day

Figure 6.15 Another use of a dummy activity.

Take another look at Brigette's college payroll activity network fragment, which you developed in Exercise 2.3 (or take a look at the model answer in Figure F.3). Redraw this as a CPM network.

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  • Welde Haylom
    Why would need dummy activity?
    3 years ago

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