Activity-on-node is a project management term that refers to a precedence diagramming method which uses boxes to denote schedule activities. These various boxes or “nodes” are connected from beginning to end with arrows to depict a logical progression of the dependencies between the schedule activities. Each node is coded with a letter or number that correlates to an activity on the project schedule.
Typically, an activity-on-node diagram will be designed to show which activities must be completed in order for other activities to commence. This is referred to as “finish-to-start” precedence – meaning one activity must be finished before the next one can start. In the diagram below, activities A and D must be done so that activity E can begin. It is also possible to create other variations of this type of diagram. For example, a “start-to-start” diagram is one in which a predecessor activity must simply be started rather than fully completed in order for the successor activity to be initiated.
An activity-on-node diagram can be used to provide a visual representation of the network logic of an entire project schedule. Or, it can be used for any smaller section of the schedule that lends itself to being represented as having a defined beginning and end. To keep the logic in the diagram simple, it may be most effective to include only critical path schedule activities. The planned start date of each node may also be listed in the diagram legend in accordance with the project management timeline.
The four dependency types
Activity-on-node diagrams support four logical relationships between activities:
- Finish-to-Start (FS) – the most common: an activity cannot start until its predecessor finishes.
- Start-to-Start (SS) – an activity cannot start until its predecessor starts.
- Finish-to-Finish (FF) – an activity cannot finish until its predecessor finishes.
- Start-to-Finish (SF) – rare: an activity cannot finish until its predecessor starts.
In practice, the vast majority of real dependencies are Finish-to-Start – and that is usually a good thing. Overusing SS or FF to compress a schedule often hides risk rather than removing it.
A worked example
Consider a small project with five activities: A (3 days) is followed by B (6 days); A is also followed by C (2 days), which is followed by D (5 days); B and D both feed a final activity E (4 days).
| Activity | Depends on | Duration |
|---|---|---|
| A – Requirements | – | 3 days |
| B – Backend build | A | 6 days |
| C – Design | A | 2 days |
| D – Frontend build | C | 5 days |
| E – Integration & test | B, D | 4 days |
There are two paths: A to B to E = 13 days, and A to C to D to E = 14 days. The longer path (14 days) is the critical path, so the project cannot finish sooner. Activity B has one day of float – it can slip a day without affecting the deadline, because it is not on the critical path. This is the practical value of the technique: it shows where to focus.
Common mistakes in practice
- Confusing “critical” with “important”. A critical-path activity is simply the one with no schedule buffer; high-value work often sits off the critical path.
- Modelling preferences as dependencies. Ask whether an activity truly requires its predecessor, or whether the team just usually does it in that order.
- Trusting the software blindly. Tools calculate the critical path automatically, but if the dependencies are wrong the result is confidently wrong.
- Forgetting to update the diagram. As activities finish or slip, the critical path can move.
Activity-on-node vs. activity-on-arrow
In activity-on-node (also called the precedence diagramming method), boxes are activities and arrows are dependencies. In the older activity-on-arrow method, arrows are the activities and nodes are events. Activity-on-node became the standard because it supports all four dependency types without artificial “dummy” activities.
Activity-on-node in the PMBOK Guide
Activity-on-node – under its formal name, the precedence diagramming method (PDM) – is documented in the current PMBOK Guide, 8th edition (2025), in the Tools and Techniques section, alongside the four relationship types shown above. The guide confirms that finish-to-start is the most commonly used relationship and that start-to-finish is very rarely used. It also cautions against using multiple logical relationships between the same two activities, and against closed loops in the logic. Earlier, process-based editions covered the same technique within the Sequence Activities process.
Frequently asked questions
What is the difference between activity-on-node and activity-on-arrow?
In activity-on-node, boxes are activities and arrows show dependencies. In activity-on-arrow, arrows are the activities. Activity-on-node is the modern standard.
Is activity-on-node the same as the precedence diagramming method?
Yes. Precedence diagramming method (PDM) is the formal name; activity-on-node describes the notation.
Which dependency type is most common?
Finish-to-Start, by a wide margin.
Last updated: 4 July 2026. Written by Tom, PMP-certified since 2004.