along with earned value, represents one of the most technical aspects of
A network, or a network diagram, is a mathematical model that contains
small circles, called nodes connected by lines, called
arcs. Nodes help define a network as they are endpoints for arcs.
Arcs link nodes to one another, therefore, arcs functions as predecessors
(tasks before) and successors (tasks after).
For a simple example, review the following figure:
5.1. Basic network
Circle A represents the starting point for a project. A precedes B and B
Networks also use paths. In network terminology, my example has the path
Large paths may have smaller paths. For example, the small A-B path could
represent development of production instructions and specifications,
while small path B-C could define manufacturing a product. Large path
A-B-C defines activities to develop instructions and specifications and
manufacture a product.
In a project network that uses circles, arcs are known as
activities and nodes are known as events.
Regardless of the specific network method you use, follow these two
Arrange your project activities in the proper order.
Identify all dependencies.
Precedence Diagram Methods
general term used for any project management network applications is
precedence diagram method (PDM). The operative word is
precedence because it signifies existence of predecessor
two PDM methods, similar in concept, came into existence around the same
time (the late 1950s).
Kelly of the Remington Rand Corporation and M.R. Walker of Du Pont, in an
effort to coordinate maintenance projects in chemical manufacturing
facilities, developed the Critical Path
Although CPM and PERT are similar, there is one main difference. While
PERT emphasizes future probabilities in time estimates (optimistic,
pessimistic, and expected time), CPM relies on past history to make
Currently, two network techniques are used extensively in project
management scheduling networks: Activity-on-Arc (AOA) and
Key Network Scheduling Principles
Regardless what network scheduling technique you use, understand and
follow these key principles:
The earliest an activity can begin based on its relationship to its
The latest an activity can begin and still satisfy requirements set forth
by successor activities.
Based on the relationship with a predecessor, the earliest an activity can
Based on the relationship with a successor, the latest an activity can be
The amount of free time (slack time) an activity has without impacting due
date of the project.
A technique that works forwards, starting with the first node in the
network, to determine earliest start and finish times.
A technique that works backwards, starting with the last node in the
network, to determine latest start and finish times.
Activity-on-Arc (AOA) is also known as Activity-on-Arrow . When
you use AOA, you use arcs or arrows, called activities, to connect
events. Consequently, AOA is event-oriented. This means that all arcs or
arrows leading to (pointing to) a node must be completed before the event
(represented by a node) is considered complete.
Activity-on-Node (AON) is the most popular project management network
scheduling technique for complex, large projects. It is interesting to
note that the name and acronym (Activity-on-Node, AON) is not that widely
used. I used software that converts a Gantt chart to an Activity-on-Node
network, but calls it a PERT chart.
5.10. AON diagram
Instead of circles and labels on arcs, AON uses a user-friendly
flowcharting approach. Because of the huge emphasis on continuous process
improvement during the past few years and the use of flowcharts, AON is
preferred because of its clarity and similarity to other tools.
When you use AON, there are no restrictions regarding the two lines
that connect two boxes. There is no need for dummy activities. An
activity exists inside a construct (a node/box), so nodes are more
important and arcs (lines/arrows) are less important.
Before an next activity can start, the first one must finish.
second dependency, start to start, saves time but also introduces risk.
start to start dependency is risky because if you cannot complete the
first activity (for example, you are unable to dig trenches because of
the presence of old tree roots or concrete), you waste time and money on
the second activity.