constructing a network diagram
DESCRIPTION
hftyhtyTRANSCRIPT
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BUSINESS DECISION BUSINESS DECISION ANALYSISANALYSIS
Constructing a Network Diagram
Click function key F5 to run this lecture as a Slide Show
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Are there any pre-requisites for this topic?
NO. You may have come across
terms like “critical path”
but no knowledge is
pre-supposed.
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Project management is also referred to as:
network analysis
critical path analysis
PERT - program evaluation & review technique
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OK, so what’s a project?
Well any managerial
activity can be represented as
a project.
For example:
• developing a new product or service
• marketing a new product or service
• breaking into a new market
• building a new plant
• installing a new computer system
• planning a training programme
• re-locating head office.
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What do your example
projects have in common?
Each can be decomposed into its
constituent parts, called ACTIVITIES.
We can then define a project as a collection
of related activities.
Three things are important:
• each activity takes time
• each activity uses up resources
• activities are structured.
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What gives activities
structure?
Precedence:
some activities have to be completed before others
can be started.
But that’s too vague. A roof cannot be erected unless the foundations have been dug. Obviously
true! So digging foundations must precede erecting the roof! But what about the walls?
So we refer to the immediately preceding activities.
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The first step for any project is to:
• list the constituent activities
• arrange them into a Precedence Table.
Activity Description Preceding TimeActivity (weeks)
A Survey site - 6
BDevelop initial proposal
- 8
CObtain approval of governors
A,B 12
D Choose architect C 4E Work out budget C 6F Finalise design D,E 15G Agree financing E 12H Hire contractor F,G 8
The project is to build an extension to the Recreation Centre. The table lists the activities
and their immediate
predecessors – up to the point of starting
construction.
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And here’s another project …………. “purchase of a new car”.
Activity DescriptionPreceding Activity
Duration (days)
A Decide feasibility of purchase - 3B Find buyer for existing car A 14C Decide on possible models A 1D Investigate models decided upon C 3E Discuss with knowledgeable friends C 1F Get information from dealers C 2G Put all information together D,E,F 1H Narrow down to three options G 1I Test drive all three H 3J Get warranty & finance information H 2K Choose one car 1,J 2L Compare dealers & choose one K 2M Decide upon colour etc L 4N Test drive chosen model L 1O Buy new car B,M,N 3
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Is activity B a pre-condition for starting activity I?
NO
Activity DescriptionPreceding Activity
Duration (days)
A Decide feasibility of purchase - 3B Find buyer for existing car A 14C Decide on possible models A 1D Investigate models decided upon C 3E Discuss with knowledgeable friends C 1F Get information from dealers C 2G Put all information together D,E,F 1H Narrow down to three options G 1I Test drive all three H 3J Get warranty & finance information H 2K Choose one car 1,J 2L Compare dealers & choose one K 2M Decide upon colour etc L 4N Test drive chosen model L 1O Buy new car B,M,N 3
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Is activity A a pre-condition for starting activity K?
YES
Activity DescriptionPreceding Activity
Duration (days)
A Decide feasibility of purchase - 3B Find buyer for existing car A 14C Decide on possible models A 1D Investigate models decided upon C 3E Discuss with knowledgeable friends C 1F Get information from dealers C 2G Put all information together D,E,F 1H Narrow down to three options G 1I Test drive all three H 3J Get warranty & finance information H 2K Choose one car 1,J 2L Compare dealers & choose one K 2M Decide upon colour etc L 4N Test drive chosen model L 1O Buy new car B,M,N 3
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Can activities E and F take place at the same time?
YES
Activity DescriptionPreceding Activity
Duration (days)
A Decide feasibility of purchase - 3B Find buyer for existing car A 14C Decide on possible models A 1D Investigate models decided upon C 3E Discuss with knowledgeable friends C 1F Get information from dealers C 2G Put all information together D,E,F 1H Narrow down to three options G 1I Test drive all three H 3J Get warranty & finance information H 2K Choose one car 1,J 2L Compare dealers & choose one K 2M Decide upon colour etc L 4N Test drive chosen model L 1O Buy new car B,M,N 3
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Precedence tables aren’t that easy to use. Is there
some other way of representing a project?
Y E S
A useful visual representation of a project is obtained by
drawing a network diagram.
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This is how we represent an activity.
activityevent of starting
an activityevent of ending
an activity
get out of bed
So somewhere in the project “Get to University Monday Morning” will be:
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Err .. but we’ll need to link activities together,
won’t we. That’s correct.
Sometimes we might want to show:
ActivityPreceding Activity
A -B AC B
A B C
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Err .. but we’ll need to link activities together,
won’t we. That’s correct.
Sometimes we might want to show:
ActivityPreceding Activity
M K,L
MK
L
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ActivityPreceding Activity
B AC A
Err .. but we’ll need to link activities together,
won’t we. That’s correct.
Sometimes we might want to show:
AB
C
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Are there any rules to follow when drawing network diagrams? YES
1. Must be drawn from left to right.
2. Must have a single starting point.
3. Also a single ending point.
4. One arrow (only) per activity.
5. Must correctly reflect the precedence table.
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There is one complication that we need to allow for.
OK, tell me the worst!!
Sometimes we have to use Dummy activities.
We use a dashed arrow to record these.
A dummy activity may be needed:
• to prevent 2 or more activities sharing the same starting and ending events.
• to maintain network logic – i.e. to ensure that the network abides by the precedence table.
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DETAILS
OK. Let’s have a go at drawing a network.
We’ll draw the one for the extension to
the recreation centre.
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DETAILS
Where do I
start?
Well, a network must have a single starting point – a circle or node.
Once the project has started which activities can be begun?
A Survey site
B Develop initial proposal
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DETAILS
A
B
So we draw arrows for activities A & B running from the starting event. Make sure that you label
the activities as you include them in the network.
What comes next?
Activity Preceding Activity
C A,B
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DETAILS
Well is this correct. Does it abide by the rules for drawing network diagrams?
That’s no problem. Just do
this.
Activity Preceding Activity
C A,B
A
BC
RULES
NO Two or more activities may not share the same starting and ending event.
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DETAILS
Use a dummy activity. Here’s one way in which it can be done. It prevents A & B from sharing the same starting and ending event.
So how do I remedy
this error?
RULES
A
B
C
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DETAILSRULES
A
B
C
Activities D & E can start when C is finished.
E
D So we’ll add these to the network.
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DETAILSRULES
The table says that F can start when D & E are
completed.
So can we do this?
A
B
CE
D
F
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DETAILSRULES
No. We’ve broken the rules again.
A
B
CE
D
F
So another dummy activity
is needed.
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DETAILSRULES
This will
work.
A
B
C
E
D
F
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RULES
Now we can add
activity G.
A
B
C
E
D
F
DETAILS
G
G can be started once E is
completed.
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RULES
That leaves
activity H.
A
B
C
E
D
F
DETAILS
G
The start of H requires the
completion of F and G.
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RULES DETAILS
A
B
C
E
D
F
G
H
So we bring the arrows for F & G into the starting event for activity H.
And this is the finished
network.
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RULES DETAILS
A
B
C
E
D
F
G
H
Always check the network against the
precedence table before continuing.
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RULES DETAILS
Right, now we’ll draw the network for the project of
buying a new car.
For this project there’s only one starting activity,
namely A
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RULES DETAILS
A
Activities B & C can be started
once A is completed.
B
C
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RULES DETAILS
A
And D, E & F can be started
when C is completed.
B
C
D
E
F
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RULES DETAILS
Activity G requires
completion of D, E & F.
So two dummy activities are needed to prevent D, E & F having the same
starting and ending events.
A B
C
D
E
F
G
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RULES DETAILS
Activity H can now be added to the
network.
A B
C
D
E
F
GH
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RULES DETAILS
I & J can now
start.
A B
C
D
E
F
GH
IJ
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RULES DETAILS
Since the start of K needs completion of I & J we need another dummy
activity.
A B
C
D
E
F
GH
IJ
K
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RULES DETAILS
Now we add
activity L
A B
C
D
E
F
GH
IJ
K
L
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RULES DETAILS
A B
C
D
E
F
GH
IJ
K
L
M
N
Activities M & N can now be
started.
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RULES DETAILS
A B
C
D
E
F
GH
IJ
K
L
M
N
That leaves
activity O.
It can be started when activities B, M & N are completed.
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RULES DETAILS
A B
C
D
E
F
GH
IJ
K
L
M
N
That leaves
activity O.
A dummy activity will be needed to separate activities M and N.
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RULES DETAILS
A
C
D
E
F
GH
IJ
K
L
M
N
The network is
now completed.
OB
But remember to
check it against the precedence
table.
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What should I now be able to do?
Given the precedence table for a project, you should be able to draw
its network diagram.
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That’s the end of this lecture. You should now explore the problems, on drawing networks, that
are available within Blackboard.