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Critical Path Analysis & PERT
Charts
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Planning and scheduling more complex projects
Critical Path Analysis and PERT are powerful tools
that help you to schedule and manage complex projects.
They were developed in the 1950s to control large
defense projects, and have been used routinely since
then.
As with Gantt Charts, Critical Path Analysis (CPA)
helps you to plan all tasks that must be completed as
part of a project. They act as the basis both for
preparation of a schedule, and of resource planning.
During management of a project, they allow you to
monitor achievement of project goals. They help you to
see where remedial action needs to be taken to get a
project back on course.
The benefit of using CPA over Gantt Charts is that
Critical Path Analysis formally identifies tasks which
must be completed on time for the whole project to be
completed on time, and also identifies which tasks can
be delayed for a while if resource needs to be
reallocated to catch up on missed tasks. The
disadvantage of CPA is that the relation of tasks to
time is not as immediately obvious as with Gantt Charts.
This can make them more difficult to understand for
someone who is not familiar with the technique.
A further benefit of Critical Path Analysis is that
it helps you to identify the minimum length of time
needed to complete a project. Where you need to run an
accelerated project, it helps you to identify which
project steps you should accelerate to complete the
project within the available time. This helps you to
minimize cost while still achieving your objective.
How to use the tool:
As with Gantt Charts, the essential concept behind
Critical Path Analysis is that you cannot start some
activities until others are finished. These activities
need to be completed in a sequence, with each stage
being more-or-less completed before the next stage can
begin. These are 'sequential' activities.
Other activities are not dependent on completion of
any other tasks. You can do these at any time before or
after a particular stage is reached. These are
non-dependent or 'parallel' tasks.
Drawing a Critical Path Analysis
Chart
Use the following steps to draw a
CPA Chart:
1. List all activities in the plan
For each activity, show the earliest start date,
estimated length of time it will take, and whether it is
parallel or sequential. If tasks are sequential, show
which stage they depend on.
For the project example used here, you will end up
with the same task list as explained in the article on
Gantt Charts (we will use the same example as with Gantt
Charts to compare the two techniques). The chart is
repeated in figure 1 below:
Figure 1. Task List: Planning a custom-written
computer project
NB: The start week shows when resources become
available. Whether a task is parallel or sequential
depends largely on context.
|
Task
|
Possible start |
Length |
Type |
Dependent
on... |
|
1. High level analysis
|
week 1 |
5 days |
sequential |
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|
2. Selection of hardware platform
|
week 1 |
1 day |
sequential |
1
|
|
3. Installation and commissioning of hardware
|
week 3 |
2 weeks |
parallel |
2
|
|
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4. Detailed analysis of core modules
|
week 1 |
2 weeks |
sequential |
1
|
|
5. Detailed analysis of supporting utilities
|
week 1 |
2 weeks |
sequential |
4
|
|
6. Programming of core modules
|
week 4 |
3 weeks |
sequential |
4
|
|
7. Programming of supporting modules
|
week 4 |
3 weeks |
sequential |
5
|
|
8. Quality assurance of core modules
|
week 5 |
1 week |
sequential |
6
|
|
9. Quality assurance of supporting modules
|
week 5 |
1 week |
sequential |
7
|
|
10.Core module training
|
week 7 |
1 day |
parallel |
6
|
|
11.Development of accounting reporting
|
week 6 |
1 week |
parallel |
5
|
|
12.Development of management reporting
|
week 6 |
1 week |
parallel |
5
|
|
13.Development of management analysis
|
week 6 |
2 weeks |
sequential |
5
|
|
14.Detailed training
|
week 7 |
1 week |
sequential |
1-13
|
|
15.Documentation
|
week 4 |
2 weeks |
parallel |
13
|
2. Plot the activities as a
circle and arrow diagram
Critical Path Analyses are presented using circle and
arrow diagrams.
In these, circles show events within the project,
such as the start and finish of tasks. Circles are
normally numbered to allow you to identify them.
An arrow running between two event circles shows the
activity needed to complete that task. A description of
the task is written underneath the arrow. The length of
the task is shown above it. By convention, all arrows
run left to right.
Where one activity cannot start until another has
been completed, we start the arrow for the dependent
activity at the completion event circle of the previous
activity.
Here the activities of 'Selecting Hardware' and 'Core
Module Analysis' cannot be started until 'High Level
Analysis' has been completed. This diagram also brings
out a number of other important points:
- Within Critical Path Analysis, we refer to
activities by the numbers in the circles at each end.
For example, the task 'Core Module Analysis' would be
called 'activity 2 to 3'. 'Select Hardware' would be
'activity 2 to 4'.
- Activities are not drawn to scale. In the diagram
above, activities are 1 week long, 2 weeks long, and 1
day long. Arrows in this case are all the same length.
- In the example above, you can see numbers above
the circles. These show the earliest possible time
that this stage in the project will be reached. Here
units are whole weeks.
As with Gantt Charts, in practice project managers
tend to use software tools like
Microsoft Project to
create CPA Charts. Not only do these ease make them
easier to draw, they also make modification of plans
easier and provide facilities for monitoring progress
against plans. Microsoft Project is reviewed at the top
of our left hand title bar.
PERT (Program Evaluation and
Review Technique)
PERT is a variation on Critical Path Analysis that takes
a slightly more skeptical view of time estimates made
for each project stage. To use it, estimate the shortest
possible time each activity will take, the most likely
length of time, and the longest time that might be taken
if the activity takes longer than expected.
Use the formula below to calculate the time to use
for each project stage:
shortest time + 4 x likely time +
longest time
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This helps to bias time estimates away from the
unrealistically short time-scales normally assumed.
Key points:
Critical Path Analysis is an effective and powerful
method of assessing:
- What tasks must be carried out
- Where parallel activity can be performed
- The shortest time in which you can complete a
project
- Resources needed to execute a project
- The sequence of activities, scheduling and timings
involved
- Task priorities
- The most efficient way of shortening time on
urgent projects.
(http://www.mindtools.com/critpath.html)
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