Scheduling a project can be daunting. Fortunately, there are tools and techniques such as the precedence diagramming method to help organize activities and get them all done on time.
Precedence diagrams are used to visualize the tasks in a project from start to finish. If you’re not familiar with the precedence diagramming method, you’ve come to the right place. Let’s take a close look at this technique and how it assists project managers when scheduling their projects.
What Is the Precedence Diagramming Method (PDM)?
The precedence diagramming method is a project planning technique that allows project managers to map out all the tasks in a project to plan the order in which they will be executed. This is done by using precedence diagrams, which are project network diagrams that show tasks, their duration and dependencies.
The precedence diagramming method (PDM) is similar to the program evaluation and review technique (PERT), activity-on-node diagramming and the critical path method (CPM), because those techniques also use project network diagrams, but each has its key differences.
What Is a Precedence Diagram?
A precedence diagram is a project management chart that represents project tasks and shows their durations and the task dependencies between them. Precedence diagrams consist of nodes, which represent tasks, and arrows that connect them to show the task dependencies so they can also be referred to as activity-on-node or AON diagrams.
However, precedence diagrams can be labor-intensive and time-consuming. You’ll need to manually adjust them every time changes are made to your project plan.
With robust project management software such as ProjectManager, you can create Gantt charts, a much more powerful alternative to precedence diagrams. Our online Gantt charts allow you to assign tasks, set task dependencies, identify the critical path and much more with just one click.
Now that we’ve defined what precedence diagrams are, let’s see how task dependencies work in the PDM.
What Are the Four Types of Precedence Diagramming Dependencies?
The precedence diagramming method uses the four types of task dependencies commonly used in project management. However, when implemented with PDM, these task dependencies are also called precedence relationships or PDM relationships.
- Finish to Start (FS): The activity cannot start until its preceding task has finished.
- Finish to Finish (FF): The activity cannot finish until its preceding task has finished.
- Start to Start (SS): The activity cannot start until its preceding task has started.
- Start to Finish (SF): The activity cannot finish until its preceding task has started.
Why Use the Precedence Diagramming Method?
A precedence diagram is a great tool to accurately develop the project schedule, keep the work on track and meet the deadline. It does this by visually representing the whole project, making it easier for project managers to plan, schedule and track tasks.
Of course, the greatest asset to using the precedence diagramming method is that it exposes task dependencies in the schedule. This helps prevent bottlenecks later in the project—and if there are changes in the schedule, the precedence diagram helps to show how those changes.
As the precedence diagram illustrates the activities and dependencies in a project, it can help show what the critical processes and activities are in the schedule. This helps to determine the critical path, which is essential for project planning and scheduling.
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Critical Path Template
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Precedence Diagram Example
A precedence diagram is a technique used in the critical path method and it’s a good way to better understand a precedence diagram by using the critical path as a precedence diagram example. In this case, the critical path diagram is used to identify the critical path in a project schedule.
The critical path is the longest sequence of tasks in a project and the tasks that must be completed on time to deliver the project as scheduled.
The critical path method template below shows two task sequences, one starts with A and the other starts with B. They both need to be done simultaneously to deliver the project. The critical path method algorithm is used to calculate the duration of the tasks and then figure out the length of the project timeline. It will also identify activities with zero slack, which is how long a task can be delayed before it impacts another task.
The example has project tasks represented as nodes, inside of which is information about those activities. Arrows indicate task dependencies.
To the left side of the precedence diagram example is a table that lists the project tasks by name, duration and dependencies. Those tasks are then shown as nodes on the precedence diagram, which shows the expected start and finish based on the earliest start (ES), earliest finish (EF), latest start (LS) and latest finish (LF).
- Earliest Start (ES): The earliest a task can start based on any preceding activities. Because activity A has no predecessors, it can start immediately so its ES is zero. On the other hand, the ES for activity C is 12 because that’s the earliest its preceding task can be completed.
- Earliest Finish (EF): The EF of a task is the sum of its ES and its duration.
- Latest Start (LS): The latest a task can start in the event its preceding task takes the longest to finish.
- Latest Finish (LF): Indicates the latest time for a task to be finished based on the sum of its latest start and its duration.
Making these distinctions will expose the critical path and, in turn, inform the estimation of a project schedule as well as highlight the work that has to be done to complete a project.
How to Draw a Precedence Diagram
1. Create a Task List
Now it’s time to draw your own precedence diagram. The first thing to do is gather tasks, list them and identify preceding tasks. At a minimum, your table should list activities and their predecessor.
2. Identify Task Dependencies
Now that you’ve identified your project tasks, it’s important to analyze them all and identify the potential task dependencies that might exist in your project. This is one of the most important steps in PDM, as failing to address any task dependency could affect the precedence diagram as a whole. It could result in task, time and schedule management consequences.
3. Put the Information Into a Diagram
Now it’s time to lay out each task or activity as a node represented by a box. Each box should have an arrow that connects it to the next step. The first task or activity will start on the left followed by the second, which will be connected by an arrow.
If the next couple of tasks or activities has the same predecessor, they will be stacked on top of one another with two arrows originating from the predecessor. When a task has two predecessors, the arrows from those predecessors both connect to the task.
4. Add Task Information to The Nodes
The final step is to indicate the activity and the duration in the node. This is done by breaking the box in half, with one half indicating the activity and the other representing the duration. Of course, it can get more complicated. Nodes can include:
- Float or slack time
- Earliest start time
- Earliest finish time
- Latest start time
- Latest finish time
Different Types of Project Network Diagrams
The precedence diagramming method is only one of many project network diagram techniques that assist with scheduling. Not all are the same, but each stems from a similar visual representation of a project schedule. More types are introduced below.
Activity-on-Node Diagram (AON Diagram)
As stated above, precedence diagrams are considered a type of activity on node diagrams because they use nodes to represent project tasks and arrows to show dependencies among them. They’re usually drawn left to right in chronological order.
Project Network Diagram (AON Diagram)
A project network diagram is used to show the order in which activities in a project are done. It comes from data collected in a work breakdown structure (WBS). It is usually drawn left to right in chronological order. A network diagram is also referred to as the AON diagram.
Critical Path Method
The critical path method uses a network diagram along with a scheduling algorithm to help project managers visualize project workflows and identify the longest sequence of activities in a project schedule in terms of duration, which is known as the critical path.
These critical activities must be completed on time or else they will delay the project delivery, which is why project managers need to pay special attention to them and allocate sufficient resources for their timely execution.
Activity-on-Arrow Diagram (AOA Diagram)
For this diagramming method, arrows represent activities and nodes show milestones in a project schedule. The relationships between the activities are shown by circles that connect one or more of the arrows. The arrow’s length is used to define the duration of the activity. This method only shows finish-to-start relationships. It’s also referred to as the activity-on-arrow (AOA) method.
Program Evaluation and Review Technique (PERT)
The PERT chart is very similar to the critical path diagram, but there are key differences between the two. First, the PERT chart is an activity-on-arrow diagram and it calculates the duration of tasks using three time estimates and a unique formula. It shows the time needed to complete the project; specifically, the time required for each of the project’s tasks, which will inform the project schedule.
Gantt Chart
The Gantt chart is a bar chart used in project scheduling. It creates a project timeline where activities are lines of varying lengths according to the duration of the task laid out in chronological order.
A Gantt chart is neither an AOA, AON nor a project network diagram, but rather an alternative to these methods that’s commonly featured in project management software thanks to its versatility as it goes beyond project scheduling.
For example, ProjectManager’s Gantt chart allows project managers to make a visual project schedule, allocate resources, track costs, assign tasks to team members and much more.
Disadvantages of Using Precedence Diagrams for Project Management
While precedence diagrams are popular project management tools, they have some disadvantages. For one, they’re time-consuming.
The process of making a precedence diagram can be tedious, especially when doing so for a larger and more complex project. Also, when working on a large project, the precedence diagram can become unruly and difficult to visualize.
It’s also not the best way to keep stakeholders informed as they might not be familiar with precedence diagrams and, therefore, not understand what they’re showing.
Another issue with precedence diagrams is that they’re prone to errors. If there are inconsistent or missing dependencies, durations or constraints, the schedule can be unreliable. The precedence diagram can also be misleading or unrealistic if it doesn’t account for risks.
How ProjectManager Helps You Schedule Projects
ProjectManager is project management software that works hand-in-hand with your precedence diagram by offering advanced scheduling and resource management features. Create dependencies, find the critical path and execute your project schedule with ease.
Plan on Gantt Charts
Once the groundwork for the precedence diagram is completed, begin inputting your tasks into our Gantt chart. You can then add the start and end dates. We automatically populate a project timeline showing a full overview of the project visually laid out for you.
Our Gantt makes it easy for you to link dependent tasks. Drag and drop one task to the other, but our tool also defines which of the four dependencies it is, allowing you to know when this work is coming up in the project so you can allocate the necessary resources to keep the team from getting blocked.
Onboard Your Team
When you’re ready to onboard the team, invite them to the software and start assigning them tasks. Add attachments, descriptions, tags and set priorities and deadlines. Monitor their progress from a high level with our real-time dashboard and report back to stakeholders with easy-to-share and filterable reports.
ProjectManager is online software that gives you live data to make better decisions as you execute your schedule. It allows you to easily plan, monitor and report on projects while giving teams a collaborative platform that gives them the tools to work better together. Use ProjectManager for your next project by taking this free 30-day trial today.
