The Critical Path Method (CPM) - Construction Project Scheduling and Control - page 54

SUPPLEMENTAL STEPS
The preceding four steps make up the “backbone” of the CPM. Following are four
additional steps that are essential for creating an effective, accurate, and successful
schedule.
5. Review and analyze the schedule
First, review the logic and make sure that every activity has the correct predecessors
and no redundant relationships. Common errors are as follows:
a. Wrong relationship : A wrong relationship occurs when the relationship is con-
nected to the wrong activity or to the appropriate activity but at the wrong end
(e.g., you connect the end of an activity when the relationship is start to start).
b. Missing relationship : A missing relationship occurs when a predecessor of an
activity is simply not shown. If an activity does not have a predecessor or a
successor, it is called a dangling activity . This is why we like to start the project
with only one activity and finish it with only one activity. Computer programs
such as Oracle Primavera P6 list all activities with either no predecessors or
no successors. We typically like to see only one of each. Project managers and
schedulers look at excessive float (explained later) as a red flag. Such excessive
float usually—but not always—indicates a dangling activity. Even with practice,
there is still a possibility of missing relationships, particularly external ones, that
is, those that tie activities under one party to other activities under other parties.
c. Redundant relationship : When, for example, C requires B and B requires A,
if we draw A and B as predecessors of C, A is a redundant predecessor of C
(Figure 4.4a). Redundant relationships are not errors. They are just a nuisance
and complication to the network.
d. Logic loop :A logic loop occurs when you leave a point on the network and
then return to it later (e.g., A requires B, but B requires A) (Figure 4.4b). This
Figure 4.4 Common scheduling errors: (a) redundant relationship; (b) logic loop