An Intro to the Precedence Diagramming Method (PDM)


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, 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.

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.

A screenshot of a status report generation screen in ProjectManager, with different filter settings that affect the generated report
ProjectManager’s Gantt charts are the perfect complement to precedence diagrams. Learn more

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 task 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 more 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.

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.

Arrow Diagramming Method (AOA Diagram)

For this diagramming method, arrows represent activities when scheduling the project. 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.

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 scheduling algorithm is often used with the program evaluation and review technique (PERT) to show the longest path of dependent activities and the time it will take to get them done.

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.

Program Evaluation and Review Technique (PERT)

As mentioned above, the PERT chart is used in conjunction with the critical path method, using statistics to analyze tasks in a project. 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.

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.

Adding precedence diagramming info to ProjectManager's Gantt chart
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’s dashboard view, which shows six key metrics on a project

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.

The post An Intro to the Precedence Diagramming Method (PDM) appeared first on ProjectManager.

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