Activity Network Diagram




Description : Activity network Diagram Also called: arrow diagram, network diagram, activity chart, node diagram, CPM (critical path method) chart , PERT (program evaluation and review technique) chart .

When to Use:

• When scheduling and monitoring tasks within a project ( pert ) or process with several dependant tasks and resources.

• When you know the steps of the project or process, their sequence and how long each step takes ( CPM ), and.

• When project schedule is critical, with serious consequences for completing the project late or significant advantage to completing the project early.

Drawing the Network Procedure :

1. 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.

2. Determine the correct sequence of the tasks. Do this by asking three questions for each task:

o Which tasks must happen before this one can begin?

o Which tasks can be done at the same time as this one?

o Which tasks should happen immediately after this one?

It can be useful to create a table with four columns —prior tasks, this task, simultaneous tasks, following tasks.

3. 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.

4. 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.

5. 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.

Problem situations:

• 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.

7. 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.

8. 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.

9. 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:

o Earliest start (ES) = the largest EF of the tasks leading into this one

o Earliest finish (EF) = ES + task time for this task






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