Also called: activity network diagram, network diagram, activity chart, node diagram, CPM (critical path method) chart
Variation: PERT (program evaluation and review technique) chart
The arrow diagram shows the required order of tasks in a project or process, the best schedule for the entire project, and potential scheduling and resource problems and their solutions. The arrow diagram lets you calculate the “critical path” of the project. This is the flow of critical steps where delays will affect the timing of the entire project and where addition of resources can speed up the project.
When to Use an Arrow Diagram
- When scheduling and monitoring tasks within a complex project or process with interrelated tasks and resources.
- When you know the steps of the project or process, their sequence and how long each step takes, and.
- When project schedule is critical, with serious consequences for completing the project late or significant advantage to completing the project early.
Arrow Diagram Procedure
Materials needed: Sticky notes or cards, marking pens, large writing surface (newsprint or flipchart pages)
Drawing the Network
- List all the necessary tasks in the project or process. One convenient method is to write each task on the top half of a card or sticky note. Across the middle of the card, draw a horizontal arrow pointing right.
- Determine the correct sequence of the tasks. Do this by asking three questions for each task:
It can be useful to create a table with four columns —prior tasks, this task, simultaneous tasks, following tasks.
Diagram the network of tasks. If you are using notes or cards, arrange them in sequence on a large piece of paper. Time should flow from left to right and concurrent tasks should be vertically aligned. Leave space between the cards.
Between each two tasks, draw circles for “events.” An event marks the beginning or end of a task. Thus, events are nodes that separate tasks.
Look for three common problem situations and redraw them using “dummies” or extra events. A dummy is an arrow drawn with dotted lines used to separate tasks that would otherwise start and stop with the same events or to show logical sequence. Dummies are not real tasks.
- Which tasks must happen before this one can begin?
- Which tasks can be done at the same time as this one?
- Which tasks should happen immediately after this one?
- Two simultaneous tasks start and end at the same events. Solution: Use a dummy and an extra event to separate them. In Figure 1, event 2 and the dummy between 2 and 3 have been added to separate tasks A and B.
- Task C cannot start until both tasks A and B are complete; a fourth task, D, cannot start until A is complete, but need not wait for B. (See Figure 2.) Solution: Use a dummy between the end of task A and the beginning of task C.
- A second task can be started before part of a first task is done. Solution: Add an extra event where the second task can begin and use multiple arrows to break the first task into two subtasks. In Figure 3, event 2 was added, splitting task A.
Figure 1: Dummy separating simultaneous tasks
Figure 2: Dummy keeping sequence correct
Figure 3: Using an extra event
- When the network is correct, label all events in sequence with event numbers in the circles. It can be useful to label all tasks in sequence, using letters.
Scheduling: Critical Path Method (CPM)
- Determine task times—the best estimate of the time that each task should require. Use one measuring unit (hours, days or weeks) throughout, for consistency. Write the time on each task’s arrow.
- Determine the “critical path,” the longest path from the beginning to the end of the project. Mark the critical path with a heavy line or color. Calculate the length of the critical path: the sum of all the task times on the path.
- Calculate the earliest times each task can start and finish, based on how long preceding tasks take. These are called earliest start (ES) and earliest finish (EF). Start with the first task, where ES = 0, and work forward. Draw a square divided into four quadrants, as in Figure 4. Write the ES in the top left box and the EF in the top right.
For each task:
- Earliest start (ES) = the largest EF of the tasks leading into this one
- Earliest finish (EF) = ES + task time for this task
Figure 4: Arrow diagram time box
LF Latest finish
- Calculate the latest times each task can start and finish without upsetting the project schedule, based on how long later tasks will take. These are called latest start (LS) and latest finish (LF). Start from the last task, where the latest finish is the project deadline, and work backwards. Write the LS in the lower left box and the LF in the lower right box.
Calculate slack times for each task and for the entire project.
- Latest finish (LF) = the smallest LS of all tasks immediately following this one
- Latest start (LS) = LF – task time for this task
Total slack is the time a job could be postponed without delaying the project schedule.
Total slack = LS – ES = LF – EF
Free slack is the time a task could be postponed without affecting the early start of any job following it.
Free slack = the earliest ES of all tasks immediately following this one – EF
Example of a completed arrow diagram
Excerpted from Nancy R. Tague’s The Quality Toolbox, Second Edition, ASQ Quality Press, 2004, pages 100-105.