Activity Network Diagram
An activity network diagram is a schedule for the completion of a
complex project.
When should we use an activity network diagram?
An activity network diagram can help you work out the most efficient
sequence of events needed to complete your project. You can use it if
you want to create a realistic project schedule by graphically showing:
- the total amount of time needed to complete the project;
- the sequence in which tasks must be carried out;
- which tasks can be carried out at the same time; and
- which are the critical tasks that you need to keep an eye on.
How do we develop an activity network diagram?
- Brainstorm for all of the tasks
necessary to complete your project, or take the output from a tree
diagram that you have already produced.
- Write each task on a Post-It™ Note or job card.
- Establish the first task that must be carried out and place its
card on the left hand side of a large work surface.
- Determine whether there are tasks that can be done at the same time,
i.e., tasks that are not dependent on this other tasks to finish before
they can be carried out. If there are, place their job cards vertically
above or below the first job card.
- Decide which is the next task that must be carried out and place
its card on the right of the first card. Determine whether there are
any tasks that can be done at the same time as this task. If there
are, place their job cards vertically above or below its job card.
- Repeat the process until all of the job cards have been arranged
in sequence or in parallel.
- Assign a number to each task and write it on the job card.
- Draw an arrow from each job card to the one that immediately follows
it.
- Estimate the elapsed time that it will realistically take to complete
each job and write this length of time at the bottom of the job card.
Do not forget to agree on a standard time unit to use throughout,
e.g. days, weeks.
- Determine the project's quickest possible implementation time by
working out its critical path. The critical path is the path of connected
activities on your diagram that has the longest completion time from
start to end.
The tasks on the critical path are the ones that should be monitored
carefully. A delay in any one of these tasks will increase the project's
total completion time and make it late. Conversely, if you want to bring
forward your project's completion time you will need to reduce the elapsed
time of one or more of the tasks on the critical path.
- The critical path can be calculated by simply adding up the times
of each activity in each path of connected activities on your diagram,
to find out which is the longest path. However, doing it this way
can quickly become confusing in larger activity network diagrams,
so there is an alternative way of determining the critical path by
calculating what is known as the 'slack' in the start times of each
task. This identifies which tasks must be completed exactly as scheduled
(these ones will be on the critical path) and those that have some
leeway.
Some tips on how to do this:
- Beginning with the first task, calculate the earliest time each
task could be started (Early Start: ES). You do this by adding the
duration of each task to the cumulative duration of its previous tasks.
This is called the forward pass.
For example:
| Previous task numbers 1 to 7 |
11 days |
| Earliest start for task number 8 |
11th day |
| Task number 8 |
3 days |
| Earliest start for task number 9 |
14th day |
Write the ES on the job card.
- Starting at the last task, calculate the latest time each task could
be started and still finish the project on schedule (Late Start: LS).
You do this by starting at the end with total implementation time
and subtracting the duration of each task from the latest start time
of its successor. This is called the backward pass.
For example:
| Total implementation time |
60 days |
| Last task (number 22) |
5 days |
| Latest start for task number 22 |
55th day |
| Task number 21 |
2 days |
| Latest start for task number 21 |
53rd day |
Write the LS on the job card.
- When you have worked your way through each task in the diagram in
this manner, you will notice that some tasks' earliest start times
will be one or more days previous to their latest start times. In
other words, you will have some leeway as to when exactly you can
decide to start these tasks. This leeway is what is known as the “slack
time” for those tasks. However, you will also notice that some
tasks' ES and LS are exactly the same. In other words, there is no
slack. Those tasks for which the ES = LS form the critical path. There
is no schedule flexibility for these tasks.
- You can now schedule the dates for your final plan.
View
sample of a complete activity network diagram
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