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The Activity Diagram

Quality Tools > Tools of the Trade > 15: The Activity Diagram


In many projects or activities, the work is divided into a set of interdependent tasks, where the order of tasks is important, but where some tasks may be performed concurrently. The Activity Diagram allows this ordering to be displayed in a simple diagram (also known as a 'PERT Chart', where PERT stands for Programmed Evaluation and Review Technique).


Computer project management software, such as Microsoft Project, can be used to draw Activity Diagrams. They can also be drawn with box-and-arrow programs, such as Visio.


How does it work?

The most common form of the Activity Diagram shows tasks in a project along with the sequential dependency between tasks with boxes for tasks and arrows to show the sequence, very much like a process flowchart.




Activity-on-node diagram

At the risk of causing a little confusion, an alternative method of showing the sequence of activities is to show the tasks as arrows, with circles to connect the ends of the arrows. This Activity-on-arrow diagram is a little more difficult than the Activity-on-node diagram described above, as 'dummy' activities (shown as dotted lines) sometimes need to be used to synchronise the real activities. It is also easier to use boxes to contain information about the individual tasks.



Activity-on-arrow diagram

How do you do it?

A simple way to build an Activity Diagram is to use 3" x 5" cards or Post-it Notes, as follows.


1. Identify tasks in the project and write these on the cards, one task per card. Keep the tasks about the same size. Thus, for example, do not have cards with tasks like 'Fit door into frame' mixed up with much larger tasks like 'build house'.


2. You can also add other information to the cards about the task, such as who will be doing it, what additional resource they need and the estimated time that will be taken.


3. Move the cards to create a left-to-right ordering of tasks, where cards further to the right are performed later. Place task cards which can be performed at the same time vertically above one another.


The advantage of using cards may now be found when you reconsider the sequence or identify new tasks that need to be inserted into the sequence. Post-it Notes are an excellent alternative as they (a) stay where they are put and (b) can be fixed to a vertical surface, allowing you and your colleagues to stand back and consider the diagram from a distance.



Organising the cards

4. The final step is to join the tasks with arrows to show the sequence. If you use Post-it Notes attached to a whiteboard or Flipchart paper, you can draw in the arrows. With cards you either need a pinboard and brown paper or will have to transcribe the boxes to paper before completing this step.


5. A 'start' and 'end' box may be added to help clarify where the project begins and ends.


Completing the Activity diagram

(Post-it Notes is a registered trade mark of 3M Corporation).


Next time: CPM: Calculating the Critical Path


This article first appeared in Quality World, the journal of the Institute for Quality Assurance

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