Identifying critical activities

The critical path identifies those activities w hich are critical to the end date of the project; however, activities that arc not on the critical path may become critical. As the project proceeds, activities w ill invariably use up some of their float and this will require a periodic recalculation of the network. As soon as the activities along a particular path use up their total float then that path will become a critical path and a number of hitherto non-critical activities will suddenly become critical.

It is therefore common practice to identify 'near-critical' paths - those whose lengths are within, say. 10-20*£ of the duration of the critical path or those with a total float of less than, say, \(Yi of the project's uncompleted duration.

The importance of identifying critical and near-critical activities is that it is they that are most likely to be the cause of delays in completing the project. We shall see, in the next three chapters, that identifying these activities is an important step in risk analysis, resource allocation and project monitoring.

For a more in-depth discussion of the role of the critical path in project monitoring, see Chapter 9.

6.18 Precedence networks

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Activity boxes allow substantial detail to be recorded on a precedence network.

Where CPM networks use links to represent activities and nodes to represent events, precedence networks use boxes (nodes) to represent activities (sometimes known as work items) and links to represent dependencies. The boxes may carry task descriptions and duration estimates and the links may contain a duration denoting a lag between the completion of one task and the start of the next.

In the notation we use here, the items indicated in the boxes are the same as those shown on the CPM network and activ ity table with the addition of activity span. Activity span is the difference between the earliest start date and the latest finish date and is a measure of the maximum time allowable for the activity.

Proponents of precedence networks claim that they are easier to draw neatly, dummy activities are v irtually redundant and it is easier for people to interpret them. Figure 6.21 shows the example project in Figure 6.20 draw n as a precedence network. As you can see. it contains much more information than the CPM network and we do not need to keep a separate activity table. The critical path through activ ities F and G is shown as a heavy line.

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Figure 6.21 A precedence network.

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Figure 6.21 A precedence network.

A further advantage of precedence networks is that they can represent parallel lagged activities. The example shown in Figure 6.16. which required the use of dummy activities in a CPM network, may be represented much more elegantly in a precedence network as shown in Figure 6.22.

Documenting amendments may take place alongside prototype testing so long as it starts at least one day later and finishes two days later.

Build

Test

Revise

prototype

prototype

specification

1

Document amendments

Figure 6.22 Parallel lagged activities in a precedence network.

Analysis of precedence networks proceeds in exactly the same ways as discussed above - we carry out a forward and backward pass to calculate earliest and latest start and finish dates and identify the critical path.

Refer back to Amanda's CPM network illustrated in Figure 6.8 and redraw it as a Exercise 6.6 precedence network.

Using the activity durations given in Table 6.5. calculate the earliest completion date for the project and identify the critical path on your network.

Table 6*5 Estimated activity durations for Amanda s network

/: s tinta ted Estimated

Table 6*5 Estimated activity durations for Amanda s network

Activity

Juration

Activity

duration

(days)

(days)

Specify overall

34

Design module C

4

Specify module A

20

Design module D

4

Specify module B

15

Code/test module A

30

Specify module C

25

Code/test module B

28

Specify module D

15

Code/test module C

15

Check specification

2

Code/test module D

25

Design module A

7

System integration

6

Design module B

6

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Project Management Made Easy

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