A project network (or project activity network) is a graphical depiction (very similar to a flow chart) that shows the sequence in which the project's terminal elements must be completed. Each terminal element represents an activity that relates to a work package at the lowest level of the work breakdown structure (WBS). However, whereas the WBS does not attempt to identify the sequence of events or the duration of any activity, the project network seeks to identify the sequence in which activities occur and the other activities (if any) on which a particular activity depends. There are a number of techniques used to create a project activity network. Some of the more commonly used techniques include Gantt charts, Critical Path Management (CPM) and the Project Evaluation and Review Technique (PERT). In some techniques, each terminal element (or activity) is represented by a node on a graph, while in others it is represented by an edge (the line connecting two nodes). Each terminal element should lie on only one path through the network. An example of a project activity network diagram is illustrated below.
An activity network diagram helps you to determine the most logical sequence of events for a project, and provides the basis of a realistic project schedule. It will enable you to calculate the total amount of time required for the project, define the order in which tasks must be carried out, and highlight those tasks that are critical to the project schedule. When the WBS has been completed you will have a list of activities at various levels of decomposition that must be completed in order to achieve the project’s objectives. You can now determine the task dependencies and the duration of each task to allow a schedule to be produced. Project management software is now available that will carry out the scheduling for you, calculate the overall time required for completing the project, and identify the schedule's critical path. All you have to do is to enter the duration of each task and identify any task dependencies. The software will produce network activity charts and Gantt charts based on the information that you provide.
Although project management software is readily available these days it is useful to understand the process by which an activity network diagram is created. The following steps can be used to carry out the process manually:
Quickly create workflow diagrams, timelines, Gantt charts, scrum boards, kanban boards, and other documentation using one of our project management templates. With our visual project management software, you can make sure that team members and other stakeholders can visualize the full scope of your project and that everyone is always on the. Dec 18, 2020 Each node in the Network Diagram view represents a task in your project. For Figure 6-13, I used Project's Zoom command (ViewOZoom) to enlarge a node so that you can see the details. A node contains the task name, duration, task ID number in the sequence of the project outline, start date, finish date, and, if assigned, the resource(s).
- Make a list of all of the tasks that must be completed (these are the work packages identified as a result of creating the work breakdown structure) and write the name of each task on a small piece of card or paper (let’s assume we will use Post-it notes, which are ideal for this purpose).
- Decide which task must be carried out first (call it Task 1), and place it on the left-hand side of a large work surface (for the purposes of this exercise I will assume we will use a whiteboard).
- Find any other tasks that can be carried out at the same time, and place them above or below Task 1 on the whiteboard. Identify these tasks (if any) as Task 2, Task 3 . . . and so on.
- Decide what the next task will be (Task n), and place it to the right of Task 1. Again, find any other tasks that can be carried out at the same time, and place them above or below Task n. Continue to number each task as you place it on the whiteboard.
- Repeat the process until all of the tasks are arranged in sequence, and in parallel, and make sure that each task has been assigned a number.
- Draw an arrow from each task to the one that immediately follows it.
- Estimate the time required for each task and write it on the Post-it (use the same unit of time for each task, i.e. hours, days, or weeks).
- Find the critical path. This is the path of connected activities which, if the duration of all of the tasks on it is added together, requires the longest time to complete.
The critical path tells you how long it will take to complete the entire project (based on current information). The tasks on the critical path, therefore, must be monitored more closely than those tasks not on the critical path, because any delays in one of these tasks will cause the critical path to increase in length (in other words, the time required to complete the project will increase). One way of reducing the overall duration of the project, of course, is to speed up the execution of one or more tasks on the critical path – assuming this is possible. For other paths through the network, each task will have some slack in the sense that it may be delayed – up to a point – because it is not on the critical path. Bear in mind however that should one or more tasks on other paths become significantly delayed, the path affected may acquire an overall duration that exceeds that of the critical path. It will therefore by definition become the new critical path.
Gantt charts, PERT and CPM are all useful techniques that provide information about project schedules in a graphical way, but they do not tell you everything. They do not tell you, for example, what resources are required for a particular task. Activity network diagrams are snapshots of the status of a project at a particular time. It will be necessary to update them as circumstances change. If a task is taking longer than expected, it may have an impact on the overall project schedule. The same is true for tasks that are completed ahead of schedule. The impact on resources will not be apparent from looking at the network diagram alone, and other tools and techniques must be used to determine where the peaks and troughs will arise in terms of the demand for critical project resources.
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.
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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)
Project Management Network Diagram Software Project
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.
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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:
- Lead
- Lag
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.
Final Thoughts
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.