Precedence networks

Activity label

Duration

Earliest start

Activity description

Earliest finish

Latest start

Latest finish

Activity span

Float

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 activity 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 virtually redundant and it is easier for people to interpret them. Figure 6.21 shows the example project in Figure 6.20 drawn 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 activities F and G is shown as a heavy line.

A | 6wks

C 1 3wks

0

Hardware design

6

6

Build hardware

9

2

8

8

11

8wks 1 2wks

5wks 1 2wks

F

10 wks

0

User

10

0

manual

10

10 wks | Owks

start

B I 6 wks

D I 4wks

H I 2 wks

f finish

0

Software design

4

4

Code software

8

9

Install & test

11

3

7

7

11

11

13

7 wks I 3 wks

7 wks I 3 wks

6 wks I 4 wks

E I 3 wks

G I 3 wks

4

File take-on

7

10

User training

13

7

10

10

13

6 wks I 3 wks

3wks I Owks

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

Estimated

Estimated

Activity

duration

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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Responses

  • gorbulas gardner
    What is recedence network in software engineering?
    3 years ago

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