Chapter 23
Lester Diagram
Abstract
This chapter describes the advantages and use of the Lester diagram, which combines the advantages of an activity on arrow network with that of an activity on node network. The data from this network can be analyzed manually or by computer, and the diagram itself can be drawn as a grid.
Keywords
Grid; Lester diagram; Network
With the development of the network grid, the drafting of an arrow diagram enables the activities to be easily organized into disciplines or work areas and eliminates the need to enter reference numbers into the nodes. Instead, the grid reference numbers (or letters) can be fed into the computer. The grid system also makes it possible to produce acceptable arrow diagrams on a computer that can be used ‘in the field’ without converting them into the conventional bar chart. An example of a computer-generated arrow diagram is shown in Fig. 23.1. It can be noticed that the link lines never cross the nodes.
A grid system can, however, pose a problem when it becomes necessary to insert an activity between two existing ones. In practice, resourceful planners can overcome the problem by combining the new activity with one of the existing activities.
If, for example, two adjoining activities were ‘Cast Column, 4 days’ and ‘Cast Beam, 2 days’, and it was necessary to insert ‘Strike Formwork, 2 days’ between the two activities, the planner would simply restate the first activity as ‘Cast Column and Strike Formwork, 6 days’ (Fig. 23.2).
While this overcomes the drafting problem, it may not be acceptable from a cost-control point of view, especially if the network is geared to an earned value analysis (EVA) system (see Chapter 32). Furthermore, the fact that the grid numbers were on the nodes meant that when it was necessary to move a string along one or more grid spaces, the relationship between the grid number and the activity changed. This could complicate the EVA analysis. To overcome this, the grid number was placed between the nodes (Fig. 23.3).
It can be argued that a precedence network lends itself admirably to a grid system, because the grid number is always and permanently related to the activity and is therefore ideal for EVA. However, the problem of the congested link lines (especially the vertical ones) remains.
Now, however, the perfect solution has been found. It is in fact a combination of the arrow diagram and precedence diagram, and like the marriage of Henry VII, which ended the Wars of the Roses, this marriage should end the war of the networks!
The new diagram, which could be called the ‘Lester’ diagram, is simply an arrow diagram where each activity is separated by a short link in the same way as in a precedence network (Fig. 23.4).
Figure 23.2
In this way, it is possible to eliminate or at least reduce logic errors, show total float and free float as easily as in a precedence network, but it has the advantages of an arrow diagram in speed of drafting, clarity of link presentation and the ability to insert new activities in a grid system without altering the grid number–activity relationship. Fig. 23.5 shows all these features.
Figure 23.3
Figure 23.6
If a line or box is drawn around any activity, the similarity between the Lester diagram and precedence diagram becomes immediately apparent (see Fig. 23.6).
Although all the examples in subsequent chapters use arrow diagrams, or precedence diagrams, ‘Lester’ diagrams could be substituted in most cases. The choice of technique is largely one of personal preference and familiarity. Provided the user is satisfied with one system and is able to extract the maximum benefit, there is little point in changing to another.
Basic Advantages
The advantages of a Lester diagram are:
1. Faster to draw than precedence diagram – about the same speed as an arrow diagram;
2. As in a precedence diagram:
a. Total float is vertical difference;
b. Free float is horizontal difference;
3. Room under arrow for duration and total float value;
4. Logic lines can cross the activity arrows;
5. Requires less space on paper when drafting the network;
6. Good for examinations due to speedy drafting and elimination of node boxes;
7. Can be updated for progress by ‘redding’ up activity arrows as arrow diagram;
8. Uses same procedures for computer inputting as precedence networks;
9. Output from computer similar to precedence network;
10. Can be used on a grid;
11. Less chance of error when calculating backward pass due to all lines emanating from one node point instead of one of the four sides of a rectangular node;
12. Shows activity as flow lines rather than points in time;
13. Looks like an arrow diagram, but is in fact more like a precedence diagram;
14. No risk of individual link lines being merged into a thick black line when printed out and
15. No possibility of creating the type of logic error often associated with ladders.
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