Project Activity Network Toolkit
1. Overview
The companion Product Breakdown Structure toolkit describes PRINCE2 Product Based Planning with its
four components:
•
Product Description: a description of the overall project deliverables. In practice the Business
case should describe what the project is to deliver, so MMU does not require a formal Product
Description separately from the Business case.
•
Product Breakdown Structure: which is the subject of a separate toolkit.
•
Product Descriptions: detailed descriptions of each product (deliverable) that goes to make up
the final product. (See PBS toolklit).
•
Product Flow diagram: this shows the sequence that the products will be delivered. This is
covered in this toolkit.
2. Why have an Activity Network?
An activity network is a diagram that shows the sequence in which tasks will take place and their interdependencies. It is therefore the basis of project planning, and it is impossible to create a full Gantt
Chart1 without having undertaken activity network planning in some form. Quite literally it is the key
tool that helps you to understand what you have to do to deliver the project.
3. What is an Activity Network
As noted above, it’s a diagram that shows activities and their dependencies. They are also known as
Network diagrams, and Activity-on-Arrow diagrams. Below is an activity network for the “project” to
make a cup of tea.
1
2
3
9
4
5
6
7
10
11
8
In this diagram 2 cannot happen until 1 has taken place, and similarly 3 can’t happen until 2 has taken
place. After this 9 and 4 are both dependent on 3, but 9 is also dependent on the chain that runs from 4
to 8. The numbered circles represent intermediate products and the arrows are the tasks that need to
happen between each product. If we add labels to the activities it becomes easier to understand the
diagram.
1
2
3
9
10
11
Put milk in cup
4
5
6
7
8
1
A Gantt Chart is the most common chart used in project management and consists of a series of horizontal bars
overlaid on a timeline, to show when activities will occur and their durations.
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The network shows the order in which tasks must happen and also where things can happen in parallel
– for example the sequence of tasks that link 4 to 8 can happen in parallel with boiling the water.
If you have completed a PBS prior to creating the activity network, the circles can be numbered using
the PBS labels. However you often find that the PBS has missed some intermediate products. For
example the tea PBS that is shown in the PBS toolkit (reproduced below) has only eight products, with
number labels added.
Cup of tea
Kettle
intermediate
3. Filled
kettle
2. Kettle
Cup
intermediate
9. Boiled
kettle
5. Washed
cup
4. Cup
10. Cup with
boiled water
Tea simple
Milk simple
6 .Teabag
8. Milk
The missing products are 1, 7 and 11. 7 could be labelled “Cup with teabag”, and 11 as “Brewed tea”.
Since all activities need to start from a point where nothing has yet happened, the “product” 1 is added
as a start point.
4. Durations
Once the tasks and dependencies have been established and drawn up, the next step is to add task
durations. This allows you to calculate how long the entire project will take. You proceed by:
•
Adding the duration of each activity
•
Putting the cumulative time so far in each circle
•
Where a circle (or node) has two or more arrows pointing to it, you take the largest time value
and ignore the lower values (this represents the critical path)
The diagram below illustrates this for the tea example.
1 [0]
2 [5]
3
[15]
9
[195]
10
[205]
11
[210]
Put milk in cup [5]
4
[20]
5
[35]
6
[45]
7
[48]
8
[58]
Node 3 has a cumulative total of 15 seconds and it takes 3 minutes (180 seconds) to boil the kettle so
node 9 has a cumulative total of 195 seconds. This is because the time for the tasks on the parallel
activity (wash cup, etc) is less than the three minutes to boil the kettle – so in spite of there being more
activities on that path, the critical path is via the boil kettle activity.
The activities on the parallel path take 48 seconds, giving 180-48=132 seconds of “slack” time – this
means there could be up to 132 seconds of delay in that set of activities without affecting the overall
project timeline. In project management terms this is called float.
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5. Using the PBS
The tea making “project” is of course a very simple example and in practice you’ll be dealing with many
times this number of activities. Whilst you can create an activity network without first completing a PBS,
for larger projects the PBS can be an aid because it allows you to break the activity networks into
smaller chunks.
Once you have completed activity networks for each “chunk” (or group of PBS products) you can then
integrate them into a larger activity network. This can be complicated because the links between the
groups may not be at convenient points at the start or end of a group’s activity network. For example
products 2, 3 and 9 are in a group from the tea example and the parallel set of activities fits between
nodes 3 and 9. Similarly the 4-5-10 group has other nodes that fit between nodes 5 and 10.
For smaller projects (of less than 50 activities) it makes sense to skip the PBS stage and just create an
activity network, but for large projects – in spite of the above complications – it makes sense to have a
PBS as well as an activity network.
6. Further work
6.1 Using the activity network to create a Gantt Chart
An activity network may be all you need to build your project plan: you now have the tasks, their
durations and their dependencies, and this is enough to build a Gantt Chart. Using our tea example, this
is what you would put into the Gantt Chart table in MS Project2:
Note that in MS Project activity totals are calculated automatically by indenting groups of activities:
here the activities are all indented under “Make a cup of tea” but as no dependencies have been
identified, MS project assumes all tasks start at the same time and happen in parallel, so the overall
duration is that of the longest task.
However, the activity network shows the links, so it’s simple to add this information to MS project:
The duration is now correctly shown as 210 and the Predecessors column shows the links, e.g. task 3 is
dependent on task 2 and task 11 is dependent on tasks 10 and 4. The Gantt Chart also shows the float
evident after task 9.
2
Note that you would not realistically use MS Project for tasks that last seconds so Project doesn’t allow entries of
less than one minute. For this example the activities have been entered in units of minutes rather than seconds!
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6.2 Beyond activity networks
One of the problems of an activity network is that it uses only one estimate of duration per task, and if
any of these duration estimates are wrong the whole project could be affected, especially if the error is
on the critical path.
One way around this is to have two or three estimates of duration: a pessimistic estimate, and
optimistic estimate and an expected estimate. This information can then be used to calculate both the
expected duration of a project (and its critical path) and also the fastest/slowest timelines. This is the
subject of PERT analysis and a separate toolkit.
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