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Activity Network: How to understand it

Quality Toolbook > Activity Network > How to understand it

What it's for | When to use it | How to understand it | Example | How to do it | Practical variations


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How to understand it

A project is composed of a set of actions or tasks which usually have some kind of interdependency. For example, before an axle can be turned, it must first be designed, the metal must be purchased, etc. This type of complex system is much easier to understand through the use of diagrams than through textual description, as actual interconnections between tasks can be shown.

The Activity Network diagram displays interdependencies between tasks through the use of boxes and arrows. Arrows pointing into a task box come from its predecessor tasks, which must be completed before the task can start. Arrows pointing out of a task box go to its successor tasks, which cannot start until at least this task is complete.


Fig. 1. The Activity Network Diagram


In a network such as this, the points where arrows meet are called nodes. Thus, as there are tasks (or activities) at these points, it is also known as an Activity-on-Node Diagram. It is usually easier to work with than the alternative Activity-on-Arrow Diagram, where the arrows represent tasks.

There are a number of attributes that can be associated with a task, such as the person doing it and the resources they need to do the job. One of the most important of these is the time required to complete each task as, once this is known, the actual calendar dates for tasks can be calculated. This is done using the Critical Path Method (or CPM). Once the start date for the overall project is known, this will give the earliest and latest start dates for each task.

The amount of time that a task can be delayed without affecting the completion time of the overall project is known as the slack time or float. Slack can either be regarded as a 'safety margin' or as wasted time. The total of all slack times for all tasks in the project gives the total time wasted, and may be reduced if the tasks can be rearranged.

When people and resources are allocated to tasks, it may also be necessary to rearrange tasks so that people do not have to work overtime to work on more than one task at once. This is called leveling or resource smoothing.

The critical path through the diagram is the sequence of tasks which have zero slack time. Thus, if any task on the critical path finishes late, then the whole project will also finish late. There is always at least one critical path. In the figure below, tasks 1 to 3 form the critical path, whilst tasks 4 and 5 may be delayed without affecting the completion date of the project.


Fig. 2. The Critical Path and Slack

It is possible to have what appears to be a task which takes no time to complete. This is called a checkpoint or milestone, and is usually included in the diagram to highlight an important point in the project.

The Activity Network can be used to identify risk in the plan. Typical areas where there is a danger of the schedule being slipped include:

  • Anywhere on the critical path.
  • Where there is a long sequence of tasks, each with a single predecessor and successor. If any task is delayed, it will delay all of its successor tasks. Also, a small risk of delay for each task adds up to a large delay for the overall task sequence.
  • Where a task has many predecessors. If any one predecessor task is delayed, then the task will also be delayed.
  • When a resource or person that may become unavailable is used in any of the above situations, the risk is compounded.
  • Where there are many tasks running at one time, particularly if there are several risky ones. A risk here is that management will not be able to cope with simultaneous failure.


Fig. 3. Risks in the Activity Network


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