nal source, or environmental hearings may need to be held before site preparation
can begin on a construction project.
.6 Constraints. Constraints are described in Section 6.1.1.4.
.7 Assumptions. Assumptions are described in Section 6.1.1.5.
6.2.2 Tools and Techniques for Activity Sequencing
.1 Precedence diagramming method (PDM). This is a method of constructing a project
network diagram using nodes to represent the activities and connecting them with
arrows that show the dependencies (see also Section 6.2.3.1). Figure 6–2 shows a
simple project network diagram drawn using PDM. This technique is also called ac-
tivity-on-node (AON) and is the method used by most project management soft-
ware packages. PDM can be done manually or on a computer.
It includes four types of dependencies or precedence relationships:
• Finish-to-start—the “from” activity must finish before the “to” activity can
start.
• Finish-to-finish—the “from” activity must finish before the “to” activity can
finish.
• Start-to-start—the “from” activity must start before the “to” activity can start.
• Start-to-finish—the “from” activity must start before the “to” activity can
finish.
In PDM, finish-to-start is the most commonly used type of logical relationship.
Start-to-finish relationships are rarely used, and then typically only by professional
scheduling engineers. Using start-to-start, finish-to-finish, or start-to-finish rela-
tionships with project management software can produce unexpected results since
these types of relationships have not been consistently implemented.
.2 Arrow diagramming method (ADM). This is a method of constructing a project
network diagram using arrows to represent the activities and connecting them at
nodes to show the dependencies (see also Section 6.2.3.1). Figure 6–3 shows a sim-
ple project network diagram drawn using ADM. This technique is also called activ-
ity-on-arrow (AOA) and, although less prevalent than PDM, is still the technique of
choice in some application areas. ADM uses only finish-to-start dependencies and
may require the use of dummy activities to define all logical relationships correctly.
ADM can be done manually or on a computer.
.3 Conditional diagramming methods. Diagramming techniques such as GERT (Graph-
ical Evaluation and Review Technique) and System Dynamics models allow for non-
sequential activities such as loops (e.g., a test that must be repeated more than once)
or conditional branches (e.g., a design update that is only needed if the inspection de-
tects errors). Neither PDM nor ADM allow loops or conditional branches.