Logic flow

The next step in the process is determining the logic flow for the project activities. This is done by determining all of the dependencies, and using them to line up the activities sequentially. The end result will be a chart which shows all of the activities, and when each will start relative to each other. Note that there are no actual dates assigned at this point, and dates are not considered in the flow. The first step is to determine what the optimal flow would be, according to the tasks and the dependencies, without worrying about other factors. Later, changes can be incorporated if necessary to take into account constraints, or resource requirements.

The output of the process can be called a project diagram, a network diagram, or a logic network depending on the source. All refer to the same thing - a diagram showing the logic flow ofthe activities.

There are two different accepted techniques for creating these diagrams, the Arrow Diagram Method (ADM) and the Precedence Diagram Method (PDM). The second is by far the more popular today, mainly because it allows much more flexibility.

In the first method, Arrow Diagram Method (ADM), the project activities are shown on arrows that run between nodes. The nodes are effectively dummies, showing only as indications of the ends of arrows. In addition to naming the activity, it is usual to also show the duration of the activity on the arrow. Any given activity must have its source in one and only one node.

As we know, in real projects, there are many project activities that are dependent on a number of activities and yet the ADM rule seems to say that an activity can be dependent on only one. In fact, that is not what the statement is saying. Any activity can have any number of dependencies, and these all have to be shown. But the rules that apply to the ADM method. say that you have to have one arrow for each activity, and that each arrow has to start from its own node. So, sometimes you have to put in a number of dummy nodes, to allow you to show all the dependencies.

Figure 3

The name of the activity and the duration are specified on the arrow. Nodes delineate ends ofthe arrow. Only one arrow can join two nodes. If two activities can occur in parallel, a dummy node must be added to terminate the second activity.

In the Precedence Diagram Method (PDM), the project activities are shown on nodes that are interconnected by arrows, which show the dependencies:

Figure 4

PDM puts the activity in the node. This technique also allows you to specify not only FS dependencies, but also SS, SF and FF dependencies. This is useful, because in real projects we have all of these. The ADM method does not really allow for this.

PDM also allows many arrows to flow into one box, although it is preferred that these be drawn clearly so that it is clear which arrow is going to which box. Sometimes problems can arise in reading diagrams with multiple arrows that cross) The diagram can be drawn your network the way an electrical circuit is drawn, with little blips to show the point at which one line crosses another.

In order to illustrate the techniques let us draw the ADM and PDM for a project with two activities, to program a module for a service order system and test/debug.

The project has two activities:

1. Program an order module

2. Test and revise

We have some duration information on these activities: Programming takes 20 hours. Testing takes 10 hours.

We also have some dependency information about the activities. Testing cannot start until 4 hours after programming starts Testing cannot finish till 5 hours after programming finishes

Using the ADM technique, the logic diagram would be:

In order to show all of the information we must split each activity into two parts and add a dummy activity to show the dependency. Dummy activities MUST have 0 duration.

Using the PDM method, the network looks simpler, even though it shows the same information:

Figure 6

Let's look at a network that is a bit more complex, first in ADM form,

Let's look at a network that is a bit more complex, first in ADM form,

Then PDM a network that is a bit more complex in first ADM form
Figure 8

Let's impose an additional condition on this network:

Suppose that establishing customer requirements can't start without initial data on the performance of the current network and it will take us 2 days to measure this.Let's see what then happens to the diagrams:

The ADM diagram becomes:

Verify initial current net

Establish customer

Confirm equipment availability

And the DM becomes:

Confirm Redesign interdependencies network

Establish customer

Confirm Redesign interdependencies network

Determine performance specifics for New equipment

Verify current performance >

Establish delivery schedules

Fieure 9

Verify initial current net

Verify current performance >

Establish delivery schedules

Determine performance specifics for New equipment

Fieure 9

Initial 8 nodes and 11 arrows become 9 nodes and 12 arrows.

Figure 10

Thus both techniques can be used. But it complex information using PDM.

is cleaner to show the more

Once the structure of the diagram has been created, the next task at hand is to determine the actual timing of each of the activities. This is done first in general terms. Later the calendar can be added to determine the exact dates. But initially we can work with activities starting on day x of the project, and completing on day x+d, where d is the duration of the activity. This will give the eventual duration for the full project, at least until problems with resources, or constraints or the addition of contingency cause it to change.

Using the sample of a logic network shown in Figure 11, we can work initially on finding the early dates - the earliest date at which each activity can start, and thus the early finish for each activity.

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