An Intro to Precedence Diagrams & the Precedence Diagramming Method

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Scheduling a project can be daunting. Fortunately, there are tools and techniques to help organize activities and get them all done on time. For example, there is the precedence diagram, which is 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 a Precedence Diagram?

A precedence diagram is a visual tool representing the schedule of a project that clearly identifies events, activities and tasks that are dependent on another to start or finish.

The precedence diagram is used to show critical tasks, noncritical tasks and slack time. As noted, the precedence diagram will illustrate the relationship of the task to other tasks. It also allows for a what-if, worst-case, best-case and most likely scenario.

Gantt charts are the perfect complement to precedence diagrams. Learn more

Partial Precedence Diagrams

One can also create a partial precedence diagram to expose only a piece of the project. These partial precedence diagrams can be connected with one another in what’s called precedence diagram dismantling. This can provide a more detailed look into the operational structure of a specific phase of the project, which assists with estimating whether the project is staying on track.

Both diagrams use boxes or nodes representing project activities. They are then connected by arrows, which are used to illustrate if they’re dependent on one another. The precedence diagram is sometimes referred to as an activity-on-node (AON) or AON method.

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.

Of course, the greatest asset to using the precedence diagramming method is that it exposes the dependencies in the schedule. This will prevent bottlenecks later in the project—and if there are changes in the schedule, the precedence diagram helps to show how those changes will impact the schedule.

Because 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 an essential scheduling tool.

What Are the Four Precedence Diagramming Dependencies?

In order to master the precedence diagramming method, you have to master dependencies. A dependency is a task dependent on another task in order to get done. There are four types of dependences, which are outlined below.

1. Mandatory Dependency

A mandatory dependency is an unavoidable dependency, such as adding water to the pool being built. One can’t add the water first and build the pool around it.

2. Discretionary Dependency

A discretionary dependency helps to optimize resources. There’s no hard logic to building the sides of the pool. Starting with one side is just as likely and important as the other. However, there might be a reason to start with one over another.

3. External Dependency

An external dependency is beyond the control of the project team. Again with the pool example, there might be the weather that prevents the starting to dig or a permit process that takes longer than expected.

4. Internal Dependency

An internal dependency is a controllable dependency. If there’s only one shovel, two people can’t dig the hole for the pool. In this case, the schedule has to split the work between the two people or buy another shovel.

What Are the Four Dependency Relations?

Every dependency can be defined in one of four ways:

  1. Finish to Start: The activity cannot start until another has finished.
  2. Finish to Finish: The activity cannot finish until another has finished.
  3. Start to Start: The activity cannot start until another has started.
  4. Start to Finish: The activity cannot finish until another has started.

How to Draw a Precedence Diagram

Now it’s time to draw your own precedence diagram. The first thing to do is gather tasks, and once all the tasks or activities are listed, make a table in which they’re all listed in sequence. A work breakdown structure template can help you organize your tasks.

At this point in the precedence diagramming method, we have a table with two columns, the first column listing the tasks or activities and the second their sequence. This second column is often called the predecessor, because it lists the task that proceeds it. The first task will not have a predecessor because it’s first and nothing comes before it.

The third and final column on the table will show the duration of each task. Now you have all the information needed to draw a simple precedence diagram.

Put the Information Into a Diagram

To start your precedence diagram, 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 have 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.

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 time
  • Earliest start time
  • Earliest finish time
  • Latest start time
  • Latest finish time

This is how a rudimentary precedence diagram is drawn.

Different Types of Visual Diagrams

The precedence diagramming method is only one of many diagramming 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 is 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 by each of the project’s tasks, which will inform the project schedule.

How ProjectManager.com Helps You Schedule Projects

ProjectManager.com is a 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. 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.

Gantt chart timeline
Our Gantt makes it easy for you to link those dependent tasks. Drag and drop one task to the other, but even better our tool will define 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, set priority and deadlines. Monitor their progress from a high level with our real-time dashboard and report back to stakeholders with easy-to-share filterable reports.

ProjectManager.com is a cloud-based 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.com for your next project by taking this free 30-day trial today.

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