Basic Networks - Construction Project Scheduling and Control - page 38

7. In many cases of hand drawing, you will need to redraw the network after
your first attempt. Although your first attempt may be logically correct, it may
look awkward and confusing. Redraw it to minimize lines crossing and relocate
activities to be as near as possible to their predecessors and successors.
In most of the preceding points, the word improper may not mean “incorrect.”
However, following the preceding guidelines will result in a better and clearer drawing.
COMPARISON OF ARROW AND NODE NETWORKS
From our discussion of arrow and node diagrams, we find that node networks have
the following four advantages over arrow networks:
1. Node networks are easier to draw.
2. Node networks do not require dummy activities to fix the activity identity
problem or to straighten out the logic.
3. Node networks can accommodate lags between activities without the addition
of more activities.
4. In addition to the preceding advantages, node diagrams can be taken to an
advanced stage in which they represent not just the traditional finish-to-start
relationship but also three other relationships (start to start, finish to finish,
and start to finish). This subject is discussed in detail in Chapter 5. However,
we mention it here as one more advantage of node networks. Arrow networks
can represent only the traditional finish-to-start relationship.
In fairness to arrow networks, they have one important advantage over node net-
works: they depict activities as well as events. Node networks do not contain provisions
for events. This point is circumvented in node diagrams by creating milestone “activ-
ities” (such as in computer programs) with a zero duration to represent an important
event such as the substantial completion of a facility. Generally, milestones in node
networks work better than event nodes in arrow networks. Milestones are tied to the
start or end of the specific activity or activities that we are considering. An event node
in arrow networks may be tied to several events that represent the end and/or start
of several activities when we are focusing on just one of these events. To understand
this point, consider the event of the completion of activity K in example 3.7. This
event is shown in the arrow network of Figure 3.7 as node 70. However, node 70 also
represents the completion of activity L and the start of activity M. These three events
may not—and usually do not—happen at the same time. Thus, the calculated event
time for node 70 may not represent what we are looking for. In node networks, we
create a milestone activity and tie it directly to the end of activity K. In this case, it
will reflect the event of completion of activity K only. In addition, contrary to arrow
networks that have a large number of event nodes, a user of node networks can decide
to create only a few milestone activities (events, in actuality), which give them better
visibility and more focus.