| The critical path is the series of activities
which represent the shortest time in which a project or
task can be completed. It is identified and calculated by
following the logic links between the various activities.
Nowadays, there are many planning software packages that
carry out this exercise in a matter of seconds.
However, we would like to explain and demonstrate by the
use of a few graphic charts the development of a simple
programme, its critical path and the float on non-critical
activities. For this demonstration we have chosen an ordinary
task and called it ‘The Sunday Lunch’; and associated
this with the development of a programme and critical path
for a construction project.
On a particular Sunday, the ‘Gibson family’,
want to have their Sunday dinner before watching their favourite
football team, Melchester Rovers, on TV at 2pm.
The first chart, fig. 1 below, shows the ‘Family’s
Desire’, which we have related to being ‘The
Contract’ on a construction project.
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| The next chart, fig. 2, is the ‘Menu’.
This can be associated with ‘The Design’ on
a construction project. |
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| Things now start to develop and the next chart, fig. 3,
is titled ‘Mum’s Instructions’. This is
the development of the ‘Menu’ (or ‘Design’),
and can be associated with using the ‘Specifications’
on a project. |
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| ‘Now the Activity Begins’, is the title of the
next chart, fig. 4 below. This is where ‘The Contractor
goes to work’, and plans what needs to be done. |
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| Figure 5 below, is a graphic representation of the planned
activities as listed in Fig.4. The activities are all consecutive,
and broadly speaking, represent the Critical Path to complete
the Sunday Dinner to enable the Gibson family to watch football
on TV, in accordance the ‘Family’s Desire’,
in fig. 1. At this stage you will note that there are no time
periods or durations on the activities. |
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| At this stage the plan needs to be scheduled. To do this
we need to, A) assign time to the activities, and B), look
at the resources available. Figure 6 below, shows how long
each of the tasks will take. As you will see two of the tasks
on fig.5, ‘Cook Food’ and ‘Serve & Eat’
have been expanded into other activities. For example, ‘Cook
Food’ has now been expanded into, ‘Roast Beef’,
‘Potatoes’, Green Beans’, ‘French
Bread’ and ‘Mr. Gibson’s Frozen Lemon Pie’.
Each of these activities has a different time period or duration. |
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| Figure 7 below, shows the resources that are available to
carry out the ‘Cook Food’ activities. |
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| The next chart, figure 8, is titled ‘The Programme’,
and can be aligned to a typical ‘Contractor’s
As Planned Programme’. As you can see, the tasks or
activities now have logic links between them. For instance,
‘Mum’s Okay’, follows the preparation and
completion of the ‘Shopping List’. You will also
note that the five number ‘Cook Food’ activities
can be carried out in parallel, and are therefore concurrent. |
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| On the next chart, fig. 9, we have highlighted the Critical
Path. This is the shortest time that all the tasks or activities
can be completed, and as you can see in the ‘Cook Food’
activities, the Critical Path runs through the ‘Cook
Roast Beef’ activity, which has a time period, or duration,
of 90 minutes. |
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You will also see from the above chart, that the four
other ‘Cook Food’ activities have a lesser duration
than ‘Cook Roast Beef’. For example, ‘Prepare/Cook
Potatoes’ has a duration of 30 minutes, which is 60
minutes less than the ‘Cook Roast Beef’ activity.
This means that ‘Prepare/Cook Potatoes’ can
be started up to 60 minutes later than ‘Cook Roast
Beef’, without delaying the overall completion of
the Gibson family’s ‘Sunday Dinner’, in
time for them to watch their favourite football team, Melchester
Rovers, on TV at 2pm. This 50 minute period of time is known
as ‘float’, i.e. the period of time an activity
can be delayed without affecting the project end date.
The as planned programme is fine, but similar to what occurs
on a construction project, actual events may or may not
affect the completion, i.e. the Gibson family sitting down
to watch TV at 2 p.m. Lets look at what happens.
Before starting the cooking phase, ‘Mum’ changed
her mind on the potatoes, she decided that instead of new
potatoes she was going to have roast potatoes. In effect,
a ‘variation’, similar to what happens on a
construction project. The roast potatoes will take an extra
20 minutes to cook, but as explained in 12 above, ‘Prepare/Cook
Potatoes’ has a float of 60 minutes. Therefore, this
change can be accommodated in the programme without delaying
the completion time. The critical path in the ‘Cook
Food’ activities remains through the ‘Cook Roast
Beef’. See fig. 10 below.
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| The next event that happens, is that during the Cooking
phase, when cooking is in progress with the Roast Beef, Potatoes
and Beans, but before cooking of the Lemon Pie starts, Dad
decides that he wants the Roast Beef cooked ‘well done’.
This will take a further 30 minutes, and as this activity
is on the critical path, it will cause a delay to completion
of the meal. Mum declares that as Dad made the change, then
he is responsible for the delay in completing the meal, and
sitting down to watch TV 30 minutes late. See figure 11 below. |
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| But this is not the end of it. Twenty minutes after Dad
changed the roast beef to being cooked well done, Mum takes
the Lemon Pie from its container and notices that the Pie
is mouldy. Its ‘use by date’ expired six months
ago. To go and buy a new Pie will take 40 minutes and will
cause a further 20 minute critical delay to completion of
the ‘Sunday Lunch’. Dad blames Mum for this delay.
This subsequent delay is concurrent with the Dad’s delay
to the Roast Beef. See fig. 12 below. |
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