Network scheduling, along with earned value, represents one of the most technical aspects of project management.
Network Basics
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:
Fig. 5.1. Basic network
Circle A represents the starting point for a project. A precedes B and B precedes C.
Networks also use paths. In network terminology, my example has the path name A-B-C.
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 principles:
- Arrange your project activities in the proper order.
- Identify all dependencies.
Precedence Diagram Methods
The 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 activities.
The two PDM methods, similar in concept, came into existence around the same time (the late 1950s).
In 1957, J.E. 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
Method (CPM).
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 estimates.
Currently, two network techniques are used extensively in project management scheduling networks: Activity-on-Arc (AOA) and Activity-on-Node (AON).
Key Network Scheduling Principles
Regardless what network scheduling technique you use, understand and follow these key principles:
· Earliest start: The earliest an activity can begin based on its relationship to its predecessor’s duration.
· Latest start: The latest an activity can begin and still satisfy requirements set forth by successor activities.
· Earliest finish: Based on the relationship with a predecessor, the earliest an activity can be completed.
· Latest finish: Based on the relationship with a successor, the latest an activity can be completed.
· Float time: The amount of free time (slack time) an activity has without impacting due date of the project.
· Forward pass: A technique that works forwards, starting with the first node in the network, to determine earliest start and finish times.
· Backward pass: A technique that works backwards, starting with the last node in the network, to determine latest start and finish times.
Activity-on-Arc
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
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.
Fig. 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.
AON Dependencies
- Finish to start.
Before an next activity can start, the first one must finish.
- Start to start.
The second dependency, start to start, saves time but also introduces risk.
The 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.