constructing a network diagram

BUSINESS DECISION BUSINESS DECISION ANALYSISANALYSIS

Constructing a Network Diagram

Click function key F5 to run this lecture as a Slide Show

Are there any pre-requisites for this topic?

NO. You may have come across

terms like “critical path”

but no knowledge is

pre-supposed.

Project management is also referred to as:

network analysis

critical path analysis

PERT - program evaluation & review technique

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.

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.

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.

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.

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

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

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

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

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.

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:

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

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

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

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.

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.

DETAILS

OK. Let’s have a go at drawing a network.

We’ll draw the one for the extension to

the recreation centre.

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

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

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.

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

DETAILSRULES

A

B

C

Activities D & E can start when C is finished.

E

D So we’ll add these to the network.

DETAILSRULES

The table says that F can start when D & E are

completed.

So can we do this?

A

B

CE

D

F

DETAILSRULES

No. We’ve broken the rules again.

A

B

CE

D

F

So another dummy activity

is needed.

DETAILSRULES

This will

work.

A

B

C

E

D

F

RULES

Now we can add

activity G.

A

B

C

E

D

F

DETAILS

G

G can be started once E is

completed.

RULES

That leaves

activity H.

A

B

C

E

D

F

DETAILS

G

The start of H requires the

completion of F and G.

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.

RULES DETAILS

A

B

C

E

D

F

G

H

Always check the network against the

precedence table before continuing.

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

RULES DETAILS

A

Activities B & C can be started

once A is completed.

B

C

RULES DETAILS

A

And D, E & F can be started

when C is completed.

B

C

D

E

F

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

RULES DETAILS

Activity H can now be added to the

network.

A B

C

D

E

F

GH

RULES DETAILS

I & J can now

start.

A B

C

D

E

F

GH

IJ

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

RULES DETAILS

Now we add

activity L

A B

C

D

E

F

GH

IJ

K

L

RULES DETAILS

A B

C

D

E

F

GH

IJ

K

L

M

N

Activities M & N can now be

started.

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.

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.

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.

What should I now be able to do?

Given the precedence table for a project, you should be able to draw

its network diagram.

That’s the end of this lecture. You should now explore the problems, on drawing networks, that

are available within Blackboard.