What activities must precede the activity being
examined? What activities can be concurrent with this
activity? What activities must follow this activity?
WORKING ACTIVITIES. Wlth respect to a
given activity, these representations indicate points in
time for the associated activities. Although the boxes
in the precedence diagram represent activities, they do
not represent time, and therefore, they are not
normally drawn to scale. They only reflect the logical
sequence of events.
CRITICAL ACTIVITIES. A critical activity
is an activity within the network that has zero float
time. The critical activities of a network makeup the
longest path through the network (critical path) that
controls the project finish date. Two slash marks
drawn through an activity connector, as shown in
figure 2-26, denote a critical path.
The rule governing the drawing of a network is
that the start of an activity must be linked to the ends
of all completed activities before that start may take
place. Activities taking place at the same time are not
linked in any way. In figure 2-24, both Activity 2 and
Activity 3 start as soon as Activity 1 is complete.
Activity 4 requires the completion of both Activities
2 and 3 before it may start.
Figure 2-26.Designation of a critical path.
Use of Diagram Connectors
Within a precedence diagram, connectors are lines
drawn between two or more activities to establish a
logic sequence. In the next paragraphs, we will look
at the diagram connectors commonly used in the
critical path method (CPM).
FINISH to START connector, as shown in
figure 2-27, is the most widely used connector
in a precedence diagram. This connector
represents the finish of one activity and the start
of another. In the activity block there are boxes
labeled EARLY FINISH (EF), LATE FINISH
(LF) and EARLY START (ES), LATE START
(LS), which is very critical in computing the
forward pass and backward pass. These terms
are covered later in this section.
START to START connector, as shown in
figure 2-28, represents activities that can either
parallel each other or that may start at the same
time or be delayed a day or two. For example,
pre-fab forms can start at the same time as
building layout starts or setting forms can begin
at the same time as excavation, but you might
want to delay the setting of forms by 1 day for
the EOs to excavate and to make sure they are
clear of where your crew is working.
FINISH to FINISH connector, as shown in
figure 2-29, is possible when the start of an
activity is independent but is not completed
before another activity is completed. Exercise
care when you use this logic connection. If
possible, the FINISH to START connector
should be used for all planning.
Figure 2-27.Finish to start connector.