What is an arrow diagram?
The arrow diagram is one of the seven management and planning tools. It shows the activities of a project as arrows between numbered events: an event is the moment when all activities leading into it are finished and the activities leaving it can start. Adding durations to the arrows and calculating the earliest and latest time of each event reveals the critical path: the chain of activities that determines when the project can finish.
Arrow diagram drawing rules
- Each activity is one arrow, from its start event to its end event; the arrow length means nothing.
- Two activities may not share both start and end event; a dummy keeps them apart.
- Use a dummy activity (dashed, zero duration) when an activity depends on only some of the activities ending at an event.
- Events are numbered so that every arrow goes from a lower to a higher number.
- There is one start event and one finish event, and no loops.
Arrow diagram example
A product launch has seven activities. Detailed design (C) needs both the market study (A) and the concept design (B), while ordering long-lead parts (D) needs only the concept design, and the marketing plan (F) only the market study. That pattern cannot be drawn with activity arrows alone, so the tool adds two dummy activities to bring A and B together before C. The critical path runs through the concept design, detailed design, prototype and launch review, for a project duration of 17 weeks; the market study has 2 weeks of slack and the marketing plan 8.
Related tools
- Critical path method: the same schedule as an activity-on-node network with floats.
- PERT chart: three-point estimates and the probability of meeting a deadline.
- Gantt chart: the plan on a calendar timeline.
How to use this Arrow Diagram tool
- List the activities. Add each activity with a code, a name and a duration.
- Enter the predecessors. For each activity, list the codes of the activities that must finish before it starts.
- Let the tool draw the network. Events, arrows and the dummy activities needed to express the dependencies are created automatically.
- Read the event times. Each circle shows the event number, the earliest time and the latest time.
- Follow the critical path. Red arrows and events have no slack: they set the project duration.
- Export. Export the arrow diagram, activities and events to PDF, PNG or Excel.
Frequently asked questions
What is an arrow diagram?
An arrow diagram (also called an activity network diagram or activity-on-arrow network) shows a project as numbered events (circles) connected by arrows, one arrow per activity. It is one of the seven management and planning tools and the original notation of CPM and PERT.
What is a dummy activity?
A dummy activity is a dashed arrow with zero duration. It does not represent work; it shows a dependency that cannot be drawn otherwise. For example, if C needs A and B but D needs only B, a dummy links the end of A to the start of C. Dummies also separate two activities that would otherwise run between the same two events.
What is the difference between activity on arrow and activity on node?
In activity on arrow (AOA), activities are arrows and nodes are events. In activity on node (AON), activities are boxes and arrows only show dependencies; no dummies are needed. AON is more common in modern software; AOA is still taught and used in the seven management tools and some industries. See the critical path method tool for AON.
What do the numbers in the circles mean?
Each event circle shows its number, its earliest time (ET: when all activities leading into it can be finished) and its latest time (LT: the latest it can occur without delaying the project). Events with ET equal to LT have no slack and lie on the critical path.
How is the arrow diagram drawn from the activity list?
The tool builds the network automatically: activities with no predecessors leave the start event, each group of predecessors that an activity needs is joined at an event, dummies are added where a predecessor feeds more than one group, and activities with no successors end at the finish event.
When should I use an arrow diagram?
Use it to plan and communicate the sequence of a project with clear dependencies, to find the critical path and to check where activities can overlap. For a calendar view of the same plan, use a Gantt chart.