Precedence diagrams explained
Blog: Monday Project Management Blog
Every project consists of a series of activities that must happen in a specific order. Look closer, and you’ll start to see hidden dependencies, or the relationships between activities that affect sequence and timing. When you’re scheduling a project, it’s important to find and account for each of these factors — and that’s where precedence diagrams come in handy.
Precedence diagramming is a mapping technique that helps you visualize the activities and dependencies in a project. Done well, it can help you create a schedule that guides resource allocation, keeps employees on track, and ensures on-time delivery. Let’s examine what a precedence diagram looks like, how to create one, and how to use it in your business.
What are precedence diagrams?
A precedence diagram is a graphic representation of a project plan. It includes the critical activities in a project, as well as the dependencies between each one. These diagrams typically feature an activity on node (AON) style that uses nodes to represent activities and arrows to represent dependencies.
If you’re developing an app, your precedence diagram might feature two key nodes: writing code and designing the graphical user interface. Each of those nodes would have arrows going to a third node representing the testing phase. The arrows indicate a dependency — in other words, both the coding and design must be complete before testing can begin.
Precedence diagram dependencies
Dependencies are a critical part of the precedence diagram. Most diagrams use four main types:
- Causal/logical: These nonnegotiable dependencies are necessary to complete an activity or the entire project. If you’re building a new product, you need a prototype before you can run tests; this creates a mandatory dependency.
- Resource: This type of internal dependency relies on constraints within your company’s control. If you have just one programmer on staff, for example, they might need to finish one project coding activity before moving to another; even if the two activities aren’t directly related, the second is dependent on the first.
- Preferential: These discretionary dependencies involve things your project team wants to do but aren’t absolutely necessary to complete the project. This might include an extra round of testing or an additional design iteration.
- External: These dependencies are based on factors your company can’t control. Examples include material shipments, government approval, or supply chain delays.
Precedence diagrams also note the relational aspect of each dependency. These relationships are based on when related activities — we’ll call them Activity A (predecessor activities) and Activity B (successor activity) — start and finish. There are four primary relationships:
- Finish to start (FS): Activity B can’t begin until Activity A is complete.
- Start to start (SS): Activity B can’t start until Activity A starts.
- Finish to finish (FF): Activity B can’t be completed until Activity A is complete.
- Start to finish (SF): Activity B can’t be completed until Activity A starts.
Using a precedence diagram
Precedence diagrams provide a visual overview of the project and the relationships between tasks. Before a project starts, you might refer to the diagram to help with scheduling activities, particularly if you’re using the critical path method. When work is underway, you can check the diagram to monitor progress at a glance.
Precedence diagrams vs. the critical path method
A precedence diagram is a standalone tool, but it can also be part of the critical path method (CPM), a project management technique that helps you create a project schedule. Briefly, this method involves:
- Listing the key activities and dependencies
- Estimating how much time each activity will take
- Sequencing activities
- Identifying the paths you can take to complete the project
- Calculate the duration of each path to find the longest one, or the critical path
If you’re using the CPM, a precedence diagram is a logical place to start. Once it’s complete, all you need to do is follow the dependency arrows to find all the possible paths through the project. After you factor in time estimates, it’s easy to identify the critical path. From there, the precedence diagram evolves into a more detailed project schedule network diagram that includes details such as slack, earliest start, and latest start. This information enables you to set the timing for goals to ensure the project is complete by the deadline.
A precedence diagram is one way to build a project schedule network diagram and locate the critical path.
How precedence diagrams affect project goal timing
When you’re setting goals and milestones for a project, you typically consider the duration of each activity, as well as its dependencies. That’s where a precedence diagram comes in handy — it makes it easier to identify the things that might affect the timing for each stage of the project. You can also spot potential roadblocks and add flex time accordingly. For example, if you know a supplier’s shipping times often vary from 2-4 weeks, you can build that time into the schedule. If the deliveries take 4 weeks, you’ll still be on track.
When there’s a change to any part of the project, the precedence diagram helps you estimate the effect on individual activities and the workflow as a whole. If your deliveries don’t arrive for 6 weeks, you can check the diagram to see which downstream tasks will be delayed. This quick, clear reference is one of the biggest benefits of the precedence diagram.
Benefits and limitations of precedence diagrams
A precedence diagram has significant benefits for project managers, but it can also require a significant amount of time and effort to create — especially for large-scale projects. Before you invest the resources to develop precedence diagrams and integrate them into your workflow, it’s important to consider the benefits and the drawbacks.
Benefits of precedence diagrams
- Makes it easier to spot dependencies and relationships
- Assists in finding potential delays or bottlenecks
- Enables efficient scheduling
- Provides a roadmap for resource allocation
- Helps you find the critical path
- Creates a quick reference tool for progress tracking
- Allows a quick response to changes or delays in the project
Limitations of precedence diagrams
- Time-consuming to create
- Requires participation from stakeholders
- Can be prohibitively complex for large projects
4 steps to create a precedence diagram
If you determine that the benefits of precedence diagrams outweigh the limitations, the real work begins. To harness the full power of the tool, start as soon as possible; that way, you have time to think through each step and gather an appropriate level of detail. If your precedence diagram becomes unwieldy, you might consider making smaller diagrams for each stage.
1. List project activities
Write down all the activities that you need to complete to finish a project, referring to your work breakdown structure, if available. Start with top-level tasks; then, work with stakeholders to refine the list until each activity can be completed by a single person or team. This collaboration is critical — managers and leaders can provide an accurate list of tasks and help you estimate the time you need for each one. If your project involves graphic design, for example, you might break it down into smaller activities: designing the initial concept, soliciting client feedback, making adjustments, and final approval.
2. Sequence activities using nodes
Place each project activity in a node, and arrange them in the order they must be completed. Keep in mind that your diagram probably won’t be a straight line; you may have parallel paths for tasks that can happen simultaneously. If you’re building a house, the roofing team could be installing shingles while the flooring team is laying tile. On a precedence diagram, these activities would be on separate parallel lines.
3. Identify and indicate dependencies
Once you’re satisfied with the sequence of activities, it’s time to note the dependencies. Look at each activity, and use arrows to indicate how it relates to other tasks. The left side of an activity node represents the starting point; the right side represents the finish. If you want to mark a finish to start (FS) dependency between two activity nodes, start the arrow on the right side (finishing point) of Activity A and place the point on the left side (starting point) of Activity B.
How to use monday.com to perfect PDM in project management
The precedence diagramming method (PDM) can involve an intricate set of dependencies and activities; monday.com can streamline the process. Tools, such as the Project Roadmap Template, can help you identify activities and set dependencies. If you’re creating a precedence diagram as part of the critical path method, the critical path for the Gantt Chart helps identify all possible paths. By entering your precedence diagram items into your Work OS, it’s easier to track the necessary adjustments if something changes.
Frequently asked questions
What is a precedence diagram?
A precedence diagram is a sequential visual representation of the individual activities that go into a project, as well as the dependencies between them. This diagram visualizes the relationships between activities to help with project scheduling, goal timing, and progress tracking.
How to draw a precedence diagram
To draw a precedence diagram, place each task into a node, and arrange them in the order they must be completed. Connect the nodes with arrows that indicate how each task affects others, paying close attention to tasks that must start or finish before others can begin.
What does a precedence diagram show?
The precedence diagram shows the workflow for an entire project, including the necessary tasks, the sequence of activities, and the dependencies that affect the schedule. It can also show the potential paths to completion and the critical path.
Streamline project management with precedence diagrams
Precedence diagrams identify critical activities and dependencies, bringing clarity to project planning. It takes time to map out the project in detail, but your efforts can pay off in efficient scheduling and on-time delivery. With the project management tools at monday.com, you can create and manage complex precedence diagrams with confidence.