I have written this article to provide in-depth explanation of Project Network Diagrams. These are used for developing project management schedule and are also called Project Schedule Network Diagrams or Logical Network Diagrams. In this article you will find different ways to draw project network diagram including Precedence Diagramming Method. You will also find explanatory pictures and examples of project network diagrams.
Project Network diagram is one of the many ways to represent project schedule. It is the most powerful way to analyze logical relationships between different activities and milestones. Some of the other popular methods to represent the project schedule are:
- Bar Charts (popularly known as Gantt Charts)
- Milestone Charts
- Project Calendars
Project Schedule Network Diagrams
Project Network Diagrams are drawn to depict project dependencies between two activities and/or milestones. An activity ‘Y’ is said to be dependent on another activity ‘X’ if Y requires an input from X. You should read Detailed Description of Dependencies and Four Types of Dependencies to understand more about project dependencies.
A Project Network Diagram can be drawn in many ways. These include:
- Precedence Diagramming Method (PDM)
- Arrow Diagramming Method (ADM)
- Conditional Diagramming Methods
Out of these PDM is the most popular. It is also called Activity on Node (AON) method. It is used by most of the modern software scheduling tools. Let us discuss it in detail.
Precedence Diagramming Method (PDM)
Predecessor Activity/Milestone – It is simply called as a Predecessor. A Predecessor logically comes before the dependent activity/milestone in a Project Network Diagram.
Successor Activity/Milestone – It is simply called as a Successor. A Successor is the dependent activity/milestone. It logically comes after Predecessor(s) in a Project Network Diagram.
Node – All the activities/milestones (Predecessors and Successors) in a Project Network Diagram are represented on a Node.
Activity – Usually an activity is drawn as a rectangle.
Milestone – Usually a milestone is drawn as a small circle. A project can have start and finish milestones as well as intermediate milestones.
Arrow – Dependencies between activities/milestones are represented as arrows. Two related nodes are separated by a unidirectional arrow. Predecessors are shown towards the arrow tail while successors are shown towards arrow head.
Sub-network – It is simply called as a subnet. It is a subdivision or a part of a complete Project Network Diagram.
Dependencies – A Successor (dependent node) can have more than one predecessors. Similarly a predecessor can have more than one successors.
Project Network Diagram Example Drawn Using PDM
The figure drawn below depicts a sample Project Network Diagram. It is drawn using PDM technique.
- There are 3 activities – A, B and C
- There are 2 Milestones – S and F
- S is the Start milestone
- F is the Finish milestone
- A has only 1 Predecessor – Milestone S
- A has only 1 Successor – Activity B
- B has only 1 Predecessor – Activity A
- B has only 1 Successor – Milestone F
- C is neither related to A nor to B
- C has only 1 Predecessor – Milestone S
- C has only 1 Successor – Milestone F
- Planned duration of each activity is mentioned in their respective Nodes
Activity Events And 4 Types of Logical Relationships in PDM
Every activity in a project schedule has two events viz. ‘Start Event’ and ‘Finish Event’. Since milestones have zero duration, their Start and Finish events are same.
PDM includes 4 types of logical relationships between two dependent activities/milestones. These are:
- FS Relationship – Start Event of a Successor is dependent Finish event of Predecessor. You can refer to Finish-to-Start (FS) Relationship to understand it thoroughly.
- SS Relationship – Start Event of a Successor is dependent Start event of Predecessor. You can refer to Start-to-Start (SS) to understand it thoroughly.
- FF Relationship – Finish Event of a Successor is dependent Finish event of Predecessor. You can refer to Finish-to-Finish (FF) Relationship to understand it thoroughly.
- SF Relationship – Finish Event of a Successor is dependent Start event of Predecessor. You can refer to Start-to-Finish (SF) Relationship to understand it thoroughly.
2 Types of Modifiers in PDM
PDM supports two Modifiers – Leads and Lags. These Modifiers do not change the nature of the logical relationships. They just change the Start and Finish events of the dependent activities. A lead accelerates the Successor activity whereas a Lag delays it.
Network Diagrams and Precedence Diagramming method is a long concept. I have split it into small articles to give it more clarity. Dependencies, Relationships, and Lead & Lag are all related concepts. You should read all the referred articles to understand the concept completely.
You can also look at Max Wideman’s Glossary for complete set of definitions.
Over To You
Do you use network diagrams while making project schedule? How do you use them? What are the other alternative methods that you use?