Project Management Network Diagrams or simply Project Network Diagrams are used for schedule preparation and analysis. They use a combination of nodes and arrows to pictorially represent activities and relationships among those activities. Since they are used for developing project schedule, they are also called Project Schedule Network Diagrams and Logical Network Diagrams.
You will find an in-depth orientation of Project Network Diagrams (PND) using pictures and examples in this article. It is mostly focused on Precedence Diagramming Method but briefly touches upon two other ways to draw a PND.
You will also find a brief description of four type project relationships, dependencies, and Lead & Lag in this article.
Project Management Network Diagrams – Definitions and Explanation
Project Network Diagrams are drawn to depict project dependencies or relationships between two activities and/or milestones.
An activity ‘Y’ is said to be dependent on another activity ‘X’ if Y requires some input from X. You can read my other articles on project dependencies and four types of dependencies in project management to understand more about the dependencies.
There are three ways to draw a Project Network Diagram. These are:
- 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 and is described in the PMBOK Guide.
Let us discuss PDM in detail.
Project Network Diagram Example
The figure drawn below depicts a sample Project Network Diagram. It is drawn using PDM technique.
In the above figure:
- There are 3 activities viz. A, B and C.
- A, B and C have a duration of 3 days, 1 day, and 5 days respectively.
- There are 2 Milestones viz. S and F.
- S and F are Start and Finish milestone respectively.
- 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
Precedence Diagramming Method (PDM)
In Precedence diagramming method, activities (or milestones) are drawn on a node and the arrows connecting the nodes depict the relationship between activities. Following are are some key points about PDM.
Activity – It is a small part of work in a project. It is represented on a Node and is usually drawn as a rectangle.
Milestone – It is a zero duration activity, which requires no work. A project can have start and finish milestones as well as intermediate milestones. It is also represented on a Node and is usually drawn as a small circle.
Predecessor Activity/Milestone – It is simply called as a Predecessor. A Predecessor logically comes before the dependent activity/milestone in a PND.
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 drawn on a Node.
Arrow – Dependencies between activities/milestones are represented by arrows. Two related nodes are separated by a unidirectional arrow. Predecessors are shown towards the arrow tail while successors are shown towards the 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.
4 Types of Logical/Project Relationships in PDM
To understand the logical relationships between two activities, we should look at each activity as a set of two distinct events. These are:
- Start Event (S)
- Finish Event (F)
So, for two related activities, we will have two sets of S and F events respectively. For these two sets, we can have four type of logical relationship between them.
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.
In all the above relationships, Predecessor’s event is written first and Successor’s event is written later e.g. FS means Finish of Predecessor and Start of Successor.
Note: Since milestones have zero duration, their Start and Finish events are same.
You can also look at Max Wideman’s Glossary for alternative definitions.
2 Types of Modifiers in PDM
PDM supports two Modifiers, which can applied to each relationship. These are:
These Modifiers do not change the nature of a logical relationship. They just change the Start and Finish events of the dependent activities. A lead accelerates the Successor activity whereas a Lag delays it. You can refer to my other articles on Project Shcedule Lead and Lag and FAQ on Lead and Lag to understand them completely.
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
Over To You
Do you use network diagrams while making project schedule? How do you use them? What other alternative methods do you use for making your project schedules?
I would love to hear from you.