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Critical Path Analysis
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Breakfast time
When cooking bacon and eggs, put the following activities in broadly the right order (A – G)
Warm the plates (4 mins)
Fry eggs (2 mins)
Heat oil in frying pan (1 min)
Heat grill (3 mins)
Grill the bacon (5 mins)
Serve breakfast (1 min)
Buy the bacon and eggs (12 mins)
State which ones have to be preceded directly by one or more other activities, by filling in
the table. Safety first so have to be in the house when grill etc are on:
ACTIVITY
A
B
C
D
E
F
G
How long does it take in total?
DURATION
12
3
1
4
5
2
1
PRECEDED BY
A
A
A
B
C
C,D,E
Bacon and eggs
Chocolates anyone?
When Cadbury’s decided to introduce Flake Bites, to compete
with Maltesers and KitKat, Management wanted to know how
long it would take to reach full scale production.
Machinery ordering and delivery will take 9 weeks, installation
takes 5 weeks, staff recruitment takes 4 weeks, training 2
weeks, suppliers need 4 weeks’ lead time, and trial production
requires 2 weeks
Preparing a network diagram
Activity
Preceded by
Duration
A
B
C
D
E
F
Machinery ordering and delivery will take 9 weeks, installation takes 5 weeks, staff
recruitment takes 4 weeks, training 2 weeks, suppliers need 4 weeks’ lead time, and
trial production requires 2 weeks
Network for Flake Bites
A
9
•1 1
•1 1
• 1
• 1
• 1
Node
A node represents:
The start of an activity
The end of an activity
Both the start and end of an activity. All
networks start and end with a node
The number represents which step it is.
Number 1 means the first step
• 1
Activity
An activity represents a task that is
undertaken, eg staff recruitment
Earliest Start Time
A
9
0
•1 1
B
4
• 91
• 1
D
5
E
2
• 1
F
2
• 1
C
4
EST - Earliest start time
EST is the earliest time an activity can start. We write this in the top right box in the node:
The EST for the first activity is now! ie 0
We start from the left and fill in, remembering we cannot start an activity until all the
preceding activities have finished, which means the EST is the longest route to a node.
Calculating EST allows a firm to organise when it needs resources, and gives the earliest
completion date for the whole project.
So for example, the earliest we can start on D (installation) is 9 weeks. This means the
earliest time Cadbury’s needs its engineers on site to install the machines is 9 weeks after
the project starts
Latest Finish Time
A
9
0
•1 1
B
4
• 91
4
• 1
D
5
E
2
• 141
F
2
• 16
1
16
C
4
LFT – Latest Finish Time
LFT shows the time by which an activity must be completed.
We write this in the bottom right box in the node; it refers to the preceding activity
We start from the right and work backwards. The final node will have a LFT of the
time a project takes, which will be the EST, in this case 16 weeks. Working back gives
14 weeks as the latest all the preceding activities (B,D and E) have to be finished by.
Calculating the LFT provides the deadlines that must be met for the project to be
completed on time. It also helps to identify the activities which have slack, or ‘float
time’, and identifies the critical path
Float time
A
9
0
•1 1
0
B
4
• 991
4
• 121
D
5
E
2
• 14
1
14
F
2
• 16
1
16
C
4
Float Time
The float time shows by how much time an activity may overrun (be delayed) before
is has an effect on either the next activity, or on the project as a whole.
It is calculated by LFT of the activity minus duration of the activity minus the EST of
the activity.
Take Activity C, which is lead time for suppliers. LFT of 14 weeks minus 4 weeks for
the activity minus EST of 0 means there is 10 weeks float, or slack
Management can decide to get a job done straightaway and then switch staff to
other activities, or take longer, or in this case order at 10 weeks for Just in Time
delivery
Critical path
A
9
0
•1 1
0
B
4
• 991
4
• 121
D
5
E
2
• 14
1
14
F
2
• 16
1
16
C
4
Critical Path
The critical path comprises those activities that take longest to complete, and so
which determine the length of the whole project.
In this case it is A, D and F, and is indicated by
A, D and F are the critical activities which must not be delayed by even 1 day –
because if they are, the whole project is delayed.
Identifying the critical path allows managers to apply management by exception,
which means management time is spent supervising the critical activities.
If a manager sees an activity might be delayed, the manager can consider moving
resources (labour or machinery) from a non-critical task
Typical business uses of CPA
• Planning a new product launch
– To minimise time-to-market
• Planning a business start-up
• In construction work, eg housebuilding, constructing
a factory
– More complex like the Olympics will use some
form of CPA, but perhaps not like this!
• In fact undertaking any project
Advantages of using CPA
1.
2.
3.
4.
It requires careful planning of the order in which events need to
occur, and how long they will take. This should make it easier to
complete a project since the planning means the project has been
carefully thought through first.
By identifying events that can be carried out simultaneously, it
shortens the length of time taken to complete a project. Very
important in the modern business focus on time-based
management
The resources needed for each activity can be ordered or hired or
transferred no earlier than needed, saving money – do not have
stock taking up room in a warehouse, new workers sitting around
drinking tea waiting to start work etc.
If the completion of an activity is delayed for some reason, the
network diagram is a good starting point for working out the
implications and deciding what to do
Disadvantages of using CPA
• A drawing for a large project (such as the Olympics) becomes
so large it may be unmanageable and very difficult to read.
These projects rely on sophisticated project management
software, not just CPA
• Drawing a diagram does not ensure the effective
management of a project. It is only a tool – poor decisions in
the planning and management of a project cannot be
overcome by a diagram
• Staff need to be consulted to make sure the estimates used in
CPA are accurate and realistic
• A focus on the time needed to complete a project may detract
from the quality
What CPA should be linked to
• Just-in-time (JIT). JIT is when a business orders
supplies just in time for them to be used, and
requires the careful planning given by CPA
• Working capital management, keeping stocks low
• Human Resource Management (HRM) planning: how
many staff are needed, with what skills, for how long