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12 CRITICAL
PATH
ANALYSIS
Objectives
After studying this chapter you should
- be able to construct activity networks;
- be able to find earliest and latest starting times;
- be able to identify the critical path;
- be able to translate appropriate real problems into a suitable
form for the use of critical path analysis.
12.0 Introduction
A complex project must be well planned, especially if a number
of people are involved. It is the task of management to
undertake the planning and to ensure that the various tasks
required in the project are completed in time.
Operational researchers developed a method of scheduling
complex projects shortly after the Second World War. It is
sometimes called network analysis , but is more usually known
as critical path analysis ( CPA ). Its virtue is that it can be
used in a wide variety of projects, and was, for example,
employed in such diverse projects as the Apollo moonshot, the
development of Concorde, the Polaris missile project and the
privatisation of the electricity and water boards. Essentially,
CPA can be used for any multi-task complex project to ensure
that the complete scheme is completed in the minimum time.
Although its real potential is for helping to schedule complex
projects, we will illustrate the use of CPA by applying it to
rather simpler problems. You will often be able to solve these
problems without using CPA, but it is an understanding of the
concepts involved in CPA which is being developed here.
12.1 Activity networks
In order to be able to use CPA, you first need to be able to form
what is called an activity network. This is essentially a way of
illustrating the given project data concerning the tasks to be
completed, how long each task takes and the constraints on the
order in which the tasks are to be completed. As an example,
consider the activities shown below for the construction of a
garage.
activity duration (in days)
A prepare foundations 7
B make and position door frame 2
C lay drains, floor base and screed 15
D install services and fittings 8
E erect walls 10
F plaster ceiling 2
G erect roof 5
H install door and windows 8
I fit gutters and pipes 2
J paint outside 3
Clearly, some of these activities cannot be started until other
activities have been completed. For example
activity G - erect roof
cannot begin until
activity E - erect walls
has been completed. The following table shows which activities
must precede which.
D must follow E
E must follow A and B
F must follow D and G
G must follow E
H must follow G
I must follow C and F
J must follow I.
We call these the precedence relation s.
The following list gives the order in which the jobs must be done:
B must be after C
A must be after B and C
D must be after B and C
E must be after D
F must be after E
G must be after A, B, C, D, E and F
Construct an appropriate activity network to illustrate this
information.
12.2 Algorithm for constructing
activity networks
For simple problems it is often relatively easy to construct activity
networks but, as the complete project becomes more complex, the
need for a formal method of constructing activity networks
increases. Such an algorithm is summarised below.
For simple problems it is often easy to construct activity networks, but as the complete project becomes morecomplex, the need for a formal method of constructing activity networks increases. Such an algorithm is summarised below.
Start Write down the original vertices and then a second copy
of them alongside, as illustrated on the right. If activity
Y must follow activity X draw an arc from original
vertex Y to shadow vertex X. (In this way you construct
a bipartite graph .)
Step 1 Make a list of all the original vertices which have no arcs
incident to them.
Step 2 Delete all the vertices found in Step 1 and their
corresponding shadow vertices and all arcs incident to
these vertices.
Step 3 Repeat Steps 1 and 2 until all the vertices have been
used.
The use of this algorithm will be illustrated using the first case
study, constructing a garage, from Section 12.1.
Original vertices A
. C
X
Y Y
X
B
A
B
C
Shadow vertices
The precedence relations are:
D must follow E
E must follow A and B
F must follow D and G
G must follow E
H must follow G
I must follow C and F
J must follow I
These are illustrated opposite.
Applying the algorithm until all vertices have been chosen is shown
below.
Step 1 - original vertices with no arcs
Step 2 - delete all arcs incident on A, B, C and redraw as
shown
Step 3 - repeat iteration
Step 1 - original vertices with no arcs
Step 2 - delete all arcs incident on E and redraw as shown
Step 3 - repeat iteration
Step 1 - original vertices with no arcs
Step 2 - delete all arcs incident on D, G and redraw as shown
Step 3 - repeat iteration
Step 1 - original vertices with no arcs
Step 2 - delete all arcs incident on F, H and redraw as shown
Step 3 - repeat iteration
Step 1 - original vertices with no arcs
Step 2 - delete all arcs incident on I and redraw as shown
A
C
A
B B
C
D
E
F
E
D
F
G
H
I
G
H
I
J J
A, B, C
E
F
E
F
G
H
I
G
H
I
J J
D D
E
D
F
D
F
G
H
I
G
H
I
J J
F
H
I
F
H
I
J J
D, G
I
J
I
J
F, H
J J
I
Exercise 12A
- Use the algorithm to find the activity network for the problem in Activity 1.
- Suppose you want to redecorate a room and put in new self-assembly units. These are the jobs that need to be done, together with the time each takes: time activity (in hrs) preceded by
paint woodwork (A) 8 - assemble units (B) 4 - fit carpet (C) 5 hang wallpaper paint woodwork hang wallpaper (D) 12 paint woodwork hang curtains (E) 2 hang wallpaper paint woodwork Complete an activity network for this problem.
- The Spodleigh Bicycle Company is getting its assembly section ready for putting together as many bicycles as possible for the Christmas market. This diagram shows the basic components of a bicycle.
Putting together a bicycle is split up into small jobs which can be done by different people. These are: time activity (mins) A preparation of the frame 9 B mounting and aligning the front wheel 7 C mounting and aligning the back wheel 7 D attaching the chain wheel to the crank 2 E attaching the chain wheel and crank to the frame 2 F mounting the right pedal 8 G mounting the left pedal 8 H final attachments such as saddle, chain, stickers, etc. 21
The following chart shows the order of doing the jobs. B must be after A C must be after A D must be after A E must be after D F must be after D and E G must be after D and E H must be after A, B, C, D, E, F and G Draw an activity network to show this information.
- An extension is to be built to a sports hall. Details of the activities are given below. time activity (in days) A lay foundations 7 B build walls 10 C lay drains and floor 15 D install fittings 8 E make and fit door frames 2 F erect roof 5 G plaster ceiling 2 H fit and paint doors and windows 8 I fit gutters and pipes 2 J paint outside 3
Some of these activities cannot be started until others have been completed: B must be after C C must be after A D must be after B E must be after C F must be after D and E G must be after F H must be after G I must be after F J must be after H Complete an activity network for this problem.
12.3 Critical path
You have seen how to construct an activity network. In this
section you will see how this can be used to find the critical
path. This will first involve finding the earliest possible start
for each activity, by going forwards through the network.
Secondly, the latest possible start time for each activity is found
by going backwards through the network. Activities which
have equal earliest and latest start time are on the critical path.
The technique will be illustrated using the 'garage construction'
problem from Sections 12.1 and 12.2.
The activity network for this problem is shown below, where
sufficient space is made at each activity node to insert two
numbers.
The numbers in the top half of each circle will indicate the
earliest possible starting time. So, for activities A, B and C, the
number zero is inserted.
Moving forward through the network, the activity E is reached
next. Since both A and B have to be completed before E can be
started, the earliest start time for E is 7. This is put into the top
half of the circle at E. The earliest times at D and G are then
both 17, and for H, 22. Since F cannot be started until both D
and G are completed, its earliest start time is 25, and
consequently, 27 for I. The earliest start time for J is then 29,
which gives an earliest completion time of 32.
7
0
0 10 2 5
0
15
10 5 8
8
Start
A D
G H
2
I (^) J Finish
F
E
2 3
0
B
C
0
0
The vertices with equal earliest and latest starting times define
the critical path. This is clearly seen to be
A E D F I J.
Another way of identifying the critical path is to define the
float time = latest start time − earliest start time.
The information for the activities can now be summarised in the
table below.
start times
activity earliest latest float
A 0 0 0 ←
B 0 5 5
C 0 12 12
E 7 7 0 ←
D 17 17 0 ←
G 17 19 2
F
H 22 24 2
I 27 27 0 ←
J 29 29 0 ←
So now you know that if there are enough workers the job can
be completed in 32 days. The activities on the critical path (i.e.
those with zero float time) must be started punctually; for
example, A must start immediately, E after 7 days, F after 25
days, etc. For activities with a non-zero float time there is scope
for varying their start times; for example activity G can be
started any time after 17, 18 or 19 days' work. Assuming that all
the work is completed on time, you will see that this does indeed
give a working schedule for the construction of the garage in the
minimum time of 32 days. However it does mean, for example,
that on the 18th day activities D and C will definitely be in
progress and G may be as well. The solution could well be
affected if there was a limit to the number of workers available,
but you will consider that sort of problem in the next chapter.
Is a critical path always uniquely defined?
Activity 2 Bicycle construction
From the activity network for Question 3 in Exercise 12A find
the critical path and the possible start times for all the activities
in order to complete the job in the shortest possible time.
Exercise 12B
- Find the critical paths for each of the activity networks shown below.
- Find the critical path for the activity network in Question 4, Exercise 12A.
- Your local school decides to put on a musical. These are the many jobs to be done by the organising committee, and the times they take: A make the costumes 6 weeks B rehearsals 12 weeks C get posters and tickets printed 3 weeks D get programmes printed 3 weeks E make scenery and props 7 weeks F get rights to perform the musical 2 weeks G choose cast 1 week H hire hall 1 week I arrange refreshments 1 week J organise make-up 1 week
8 5 8 5
5 7
7
5 11
(^3) Finish
12 0
1 2 11 0 7 5
1 2 4
10 3
10
Start^11
(a)
(b) 6
8
7
4
10 7
10
10 7 Start 0 Finish
5
3
(c)
I
C E H
A B F
G
B E
D
C F
A D (^) G
A C G
D F
B E
Finish
Start
7
K decide on musical 1 week L organise lighting 1 week M dress rehearsals 2 days N invite local radio/press 1 day P choose stage hands 1 day Q choose programme sellers 1 day R choose performance dates 12 day S arrange seating 12 day T sell tickets last 4 weeks V display posters last 3 weeks
(a) Decide on the precedence relationships. (b) Construct the activity network. (c) Find the critical path and minimum completion time.
- The activities needed to replace a broken window pane are given below. duration preceding activity (in mins) activities
A order glass 10 - B collect glass 30 A C remove broken pane 15 B, D D buy putty 20 - E put putty in frame 3 C F put in new pane 2 E G putty outside and smooth 10 F H sweep up broken glass 5 C I clean up 5 all
(a) Construct an activity network. (b) What is the minimum time to complete the replacement? (c) What is the critical path?
- Write the major activities, duration time and precedence relationship for a real life project with which you are involved. Use the methods in this chapter to find the critical path for your project.
- Consider the following activity network, in which the vertices represent activities and the the numbers next to the arcs represent time in weeks:
(a) Write down the minimum completion time of the project, if an unlimited number of workers is available, and the corresponding critical path. (b) Find the float times of activities D and B.
- A firm of landscape gardeners is asked to quote for constructing a garden on a new site. The activities involved are shown in the table.
duration preceding activity (in days) activities A prepare site 2 - B build retaining wall for 3 A patio
C lay patio * (see below) 4 A
D lay lawn 1 A E lay paths 3 A B F erect pergola, 1 A B D G trellis, etc. G prepare flower 1 A B D beds and border H plant out 3 A B D G I clean up 1 all
* Note also that the patio cannot begin to be laid
until 2 days after the start of the building of the retaining wall. (a) Construct an activity network for this problem. (b) Find the earliest and latest start time for each activity, state the minimum time for completion of the work and identify the critical path. (c) Which activities have the greatest float time? (AEB)
- At 4.30 pm one day the BBC news team hear of a Government Minister resigning. They wish to prepare an item on the event for that evening's 6 o'clock news. The table below lists the jobs needed to prepare this news item, the time each job takes and the constraints on when the work can commence. Time needed Constraints Job (in minutes) A Interview the 15 Starts at 4.30 pm resigning Minister B Film Downing St. 20 None C Get reaction from 25 Cannot start until regions A and B are completed D Review possible 40 Cannot start until replacements B is completed E Review the Minister's 25 Cannot start until career A is completed F Prepare film for 20 Cannot start until archives C and E are completed G Edit 20 Cannot start until A, B, C, D, E and F are completed
Start 0
2
7 4
4
E
F
C
0
B 2
D
0 A 2
(^6) G 4 (^4) H 5
(^3) Finish
(a) Construct an activity network for this problem and, by finding the critical path in your network, show that the news item can be ready before 6.00 pm that day.
(b) If each of the jobs A, B, C, D, E and F needs a reporter, and once a reporter has started a job that same reporter alone must complete it; explain how three reporters can have the news item ready before 6.00 pm, but that two reporters cannot. (AEB)
