Sequencing and scheduling activities

Throughout a project, we will require a schedule that clearly indicates w hen each of the project's activities is planned to occur and what resources it will need. We shall be considering scheduling in more detail in Chapter 8, but let us consider in outline how we might present a schedule for a small project. One way of presenting such a plan is to use a bar chart as shown in Figure 6.6.

The chart shown has been drawn up taking account of the nature of the development process (that is. certain tasks must be completed before others may start) and the resources that are available (for example, activity C follows activity B because Andy cannot work on both tasks at the same time). In drawing up the chart, we have therefore done two things - we have sequenced the tasks (that is, identified the dependencies among activities dictated by the development process) and scheduled them (that is, specified when they should take place). The

Separating the logical sequencing from the scheduling may be likened to the principle used in SSADM of separating the logical system from its physical implementation.

1 he bar chart does not show why certain decisions have been made. It is not clear, for example, why activity H is not scheduled to start until week 9. It could be that it cannot start until activity F has been completed or it might be because Charlie is going to be on holiday during week 8.

Task Porton 1



4 I 5









A: Andy H


C : Andy

D : Andy


E : Bil

F : Bi

G : CharWe

H : Charlie

1 Dave

Activity key: A: Overall design

B: Specify module I C: Specify module 2 D: Specify module 3 E: Code module I

Figure 6.6 A project plan as a Ixir chart.

F: Code module 3 G: Code module 2 H: Integration testing I: System testing scheduling has had to take account of the availability of staff and the ways in which the activities have been allocated to them. The schedule might look quite different w ere there a different number of staff or were w e to allocate the activities differently.

In the case of small projects, this combined sequencing-scheduling approach might be quite suitable, particularly where we wish to allocate individuals to particular tasks at an early planning stage. However, on larger projects it is better to separate out these two activities: to sequence the task according to their logical relationships and then to schedule them taking into account resources and other factors.

Approaches to scheduling that achieve this separation between the logical and the physical use networks to model the project and it is these approaches that we will consider in subsequent sections of this chapter.

6.7 Network planning models

CPM was developed by the Du Pont Chemical Company who published the method in 1958. claiming that it had saved them $1 million in its first year of use.

These project scheduling techniques model the project's activities and their relationships as a network. In the network, time flows from left to right. These techniques were originally developed in the 1950s - the two best known being CPM (Critical Path Method) and PERT (Program Evaluation Review Technique). More recently a variation on these techniques, called precedence networks, has become popular and it is this method that is adopted in the majority of computer applications currently available. All three methods are very similar and it must be admitted that many people use the same name (particularly CPM) indiscriminately to refer to any or all of the methods.

In the following sections of this chapter, we will look at the critical path method and precedence networks - a discussion of PERT will be reserved for Chapter 7 when we look at risk analysis.


6.8 Formulating a network model

The first stage in creating a network model is to represent the activities and their interrelationships as a graph. In CPM we do this by representing activities as links (arrowed lines) in the graph - the nodes (circles) representing the events of activities starting and finishing.

In Chapter 2 we saw how Amanda used her Product Breakdown to obtain an Case Study Example activity network. Figure 6.7 shows the fragment of her network that was discussed in that chapter and Figure 6.8 shows how this network would look represented as a critical path network.

Software Project Planning Activity
Figure 6.7 The IOE maintenance group accounts project activity network fragment with a checkpoint activity added.
Figure 6.8 The IOE maintenance group accounts project activity network fragment represented as a CPM network.

Constructing CPM networks

Before we look at how CPM networks are used, it is worth spending a few moments considering the rules for their construction.

CPM networks are examples of directed graphs.

A project network may have only one start node The start node (node I in Figure 6.8) designates the point at which the project may start. All activities coming from that node may start immediately resources are available - that is, they do not have to wait for any other activities to be completed.

A project network may have only one end node The end node designates the completion of the project and a project may only finish once! The end node for the project fragment shown in Figure 6.8 is the one numbered 10.

A link has duration A link represents an activity and, in general, activities take time to execute. Notice, however, that the network in Figure 6.8 does not contain any reference to durations. The links are not drawn in any way to represent the activity durations. The network draw ing merely represents the logic of the project - the rules governing the order in which activities are to be carried out.

Nodes have no duration Nodes are events and, as such, are instantaneous points in time. The source node is the event of the project becoming ready to start and the sink node is the event of the project becoming completed. Intermediate nodes represent two simultaneous events - the event of all activities leading in to a node having been completed and the event of all activities leading out of that node being in a position to be started.

In Figure 6.9 node 3 is the event that both coding and data take-on have been completed and activity program testing is free to start. Installation may be started only when event 4 has been achieved, that is, as soon as program testing has been completed.

Figure 6.9 Fragment of a CPM network.

Time moves from left to right If at all possible, netw orks are draw n so that time moves from left to right. It is rare that this convention needs to be flouted but, in any case, the arrow s on the activity lines give a strong visual indication of the time flow of the project.

Nodes are numbered sequentially There are no precise rules about node numbering but nodes should be numbered so that head nodes (those at the 'arrow* end of an activity) always have a higher number than tail events (those at the 'non-arrow' end of an activity. This convention makes it easy to spot loops.

A network may not contain loops Figure 6.10 demonstrates a loop in a CPM network. A loop is an error in that it represents a situation that cannot occur in practice. While loops, in the sense of iteration, may occur in practice, they cannot be directly represented in a project network. Note that the logic of Figure 6.10 suggests that program testing cannot start until the errors have been corrected.


Figure 6.10 A loop represents an impossible sequence.

If we know the number of times we expect to repeat a set of activities, a test-diagnose-correct sequence, for example, then we can draw that set of activities as a straight sequence, repeating it the appropriate number of times. If we do not know how many times a sequence is going to be repeated then we cannot calculate the duration of the project unless we adopt an alternative strategy such as redefining the complete sequence as a single activity and estimating how long it w ill take to complete it.

A network may not contain dangles A dangling activity such as Write user manual in Figure 6.11 cannot exist, as it would suggest there are two completion points for the project. If, in Figure 6.11 node 5 represents the true project completion point and there are no activities dependent on activity Write user manual, then the network should be redrawn so that activity Write user manual starts at node 2 and terminates at node 5 - in practice, we would need to insert a dummy activity between nodes 3 and 5 as described in Section 6.9. In other words, all events, except the first and the last, must have at least one activity entering them and at least one activity leaving them and all activities must start and end with an event.

Precedents are the immediate preceding activities In Figure 6.9, the activ ity Program test cannot start until both Code and Data take-on have been completed and activity Install cannot start until Program test has finished. Code and Data take-on can therefore be said to be precedents of Program test, and Program test is a precedent of Install. Note that we do not speak of Code and Data take-on as precedents of Install - that relationship is implicit in the previous statement.

Dangles are not allowed in activity networks. Although undesirable, they are allowed in precedence networks (discussed in Chapter 9).

Figure 6.11 A dangle.

Exercise 6.1 Take a look at the networks in Figure 6.12. State w hat is wrong with each of them and where possible redraw them correctly.


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  • Gundolpho Took-Took
    How project sequencing and scheduling performed in spm?
    4 years ago
  • cornelia zaragamba
    How are sequencing and scheduling of activities represented?
    4 years ago
  • adonay
    What is sequence ing and scheduling activities in spm?
    4 years ago
  • michael
    How the sequencing is done in management?
    4 years ago

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