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This
is a way of demonstrating how a complex project can be completed in the
shortest possible time. It identifies the critical path, these are the activities
that must be completed on time in order to delay the whole project.
Constructing a Network
We
will use the example of the tasks involved producing an advertising campaign,
they are:
- A plan the
advertising campaign;
- B make a TV
video;
- C design a
poster;
- D test market
the TV video;
- E test the
suitability of the poster;
- F present
campaign to the board of directors;
- G communicate
the campaign to all company personnel.
The
estimated length of time for each task and the order, i.e., the tasks that
depend upon others being completed are shown below.
|
Tasks
|
Order/dependency
|
Estimated
time (hrs)
|
|
A
|
Must
be done first
|
4
|
|
B
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Can
only start when A is completed
|
6
|
|
C
|
Can
only start when A is completed
|
7
|
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D
|
Can
only start when B is completed
|
8
|
|
E
|
Can
only start when C is completed
|
10
|
|
F
|
Can
only start when D and E are completed
|
9
|
|
G
|
Can
only start when D, E and F are completed
|
5
|
These
activities can be shown on a network; a generalised network is shown overleaf
followed by a network for the advertising campaign.
The
advertising network shows the order in which the tasks can be completed.
EARLIEST START TIME
If
we assume that the earliest start time for A is 0, then B and C can not start
for 4 hours (0+4), these figures are shown in the top right hand corner of
nodes 1 and 2. Task D cannot
start until A and B have been completed, this takes 10 hours (0+4+6), which
is shown in node 3. Task E
cannot start until A and C have been completed, this takes 11 hours (0+4+7),
which is shown in node 3. Task F
cannot start until E and D are completed, the EST for task F is 21
(0+4+7+10). It is important to
choose the longest route when calculating the ESTs. The route A, B, D is only 18 (0+4+6+8), the EST of the longer
route is shown in node 5.
LATEST FINISH TIME
The
next step is to calculate the latest finish time (LFT) of each task without
extending the whole project. We
must start at node 7 and work back. Task
G must be completed by the 35th hour and this is shown in node 7.
To calculate the LFT for task F, we subtract the time it takes to
complete task F from the previous LFT (35-5=30) and place it in node 6.
When choosing between two LFTs, such as for task A we choose the route
that gives the lowest LFT, in this case 4 (11-7) rather than 7 (13-6).
THE CRITICAL PATH
Once
all of the LFTs have been identified it is possible to outline the critical
path. This can be drawn through
all of the nodes where the ESTs and the LFTs are the same. This
means that there can be no delays between completing the previous tasks and
starting the next one without prolonging the advertising campaign.
The critical path in this case is A, C, E, F, G is shown by striking
two short lines across the critical activities.
If the manager sees that there is a possibility of a task on the
critical path overrunning he or she can considering shifting labour across
from a non-critical task.
THE FLOAT
B
and D could be delayed for up to 3 hours without affecting the total time of
the task, this is called the float. The
total float is found by subtracting the EST and the duration from the
LFT, so for task B it would be 3 (13-6-4), this is the total float up to that
activity. The free float
is found by subtracting the EST at the start of the task and the duration
from the EST at the end, so for task B it would be 0 (10-6-4).
This is the free float for the task, i.e., its delay will interfere
with other tasks, in this case D.
|
Task
|
Duration
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EST
|
LFT
|
Total
Float
|
Free
Float
|
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A
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4
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0
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4
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0
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0
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B
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6
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4
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13
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3
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0
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C
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7
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4
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11
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0
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0
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D
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8
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10
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21
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3
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3
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E
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10
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11
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21
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0
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0
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F
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9
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21
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30
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0
|
0
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G
|
5
|
30
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35
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0
|
0
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Advantages of Network Analysis
- Reduce lost time
between tasks, ensuring a smooth running of projects.
- By identifying
projects that can be carried out simultaneously it should reduce the
overall time of the project.
- The resources needed
for each activity should be ordered for no earlier than the EST for that
task. This should reduce the
cash outflows and ensure the working capital tied up in the project is
minimised. This will also
assist JIT production if used.
- If the project is
delayed, the network is a good starting point for working out the
implications.
Disadvantages of Network Analysis
- A very complex
project drawing becomes unmanageable, however the introduction of ICT has
reduced this problem.
- The network is only
as effective as the staffs commitment to it.
 Further
Reading
How
businesses use critical path analysis
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