Pert, cpm & gert

Network Scheduling,
Planning & Controlling
Techniques

Introduction
 Schedule converts action plan into operating time table
 Basis for monitoring and controlling project
 Scheduling more important in projects than in production,
because unique nature
 Sometimes customer specified/approved requirement.
 Based on Work Breakdown Structure (WBS)

 Graphical portrayal of activities and event
 Shows dependency relationships between tasks/activities in a
project
 Clearly shows tasks that must precede (precedence) or follow
(succeeding) other tasks in a logical manner
 Clear representation of plan – a powerful tool for planning
and controlling project
Network

Networking Techniques
CPM GERTPERT

History Of PERT/CPM
Developed by
the US Navy for
the planning
and control of
the Polaris
missile program
The emphasis
was on
completing the
program in the
shortest possible
time.
PERT Developed by
Du Pont to solve
project
scheduling
problems
The emphasis
was on the
trade-off
between the
cost of the
project and its
overall
completion time
CPM

Why PERT/CPM?
 Prediction of deliverables
 Planning resource requirements
 Controlling resource allocation
 Internal program review
 External program review
 Performance evaluation
 Uniform wide acceptance

Applications Of PERT/CPM Techniques
1
• Construction of a Dam or Canal
• Construction of a building or highway
2
• Maintenance or Overhaul of aircrafts
• Space Flights
3
• Designing a Prototype of a Machine
• Development of Supersonic Planes

Steps in PERT/CPM
4. Controlling
3. Allocation Of Resources
2. Scheduling
1. Planning

Need of PERT/CPM
 Prediction of deliverables
 Planning resource requirements
 Controlling resource allocation
 Internal program review
 External program review
 Performance evaluation
 Uniform wide acceptance

PERT
Project Evaluation
and Review
Technique (PERT)
• U S Navy (1958) for the POLARIS
missile program
• Multiple task time estimates
(probabilistic nature)
• Activity-on-arrow network
construction
• Non-repetitive jobs (R & D work)

 PERT is based on the assumption that an activity’s duration
follows a probability distribution instead of being a single value
 Three time estimates are required to compute the parameters of
an activity’s duration distribution:
 pessimistic time (a) - the time the activity would take if things
did not go well
 most likely time (m ) - the consensus best estimate of the
activity’s duration
 optimistic time (b) - the time the activity would take if things
did go well
Mean (expected time):te = a + 4m + b
6
Variance: V =
b- a
6
2
PERT

Use of PERT
 In construction activities
 Transportation activities
 In oil refineries
 Computer system-
 For manufacturing electric generator machines
 Medical and surgical sector
 Library activities

Importance of PERT System
 Reduction in cost
 Saving of time
 Determination of activities
 Elimination of risk in complex activities –
 Flexibility
 Evaluation of alternatives-
 Useful in effective control-
 Useful in decision making
 Useful is research work

Critical path
 Those activities which contribute directly to the overall duration of the
project constitute critical activities, the critical activities form a chain
running through the network which is called critical path.
 Critical event : the slack of an event is the difference between the latest &
earliest events time. The events with zero slack time are called as critical
events.
 Critical activities : The difference between latest start time & earliest start
time of an activity will indicate amount of time by which the activity can be
delayed without affecting the total project duration. The difference is
usually called total float. Activities with 0 total float are called as critical
activities

Critical Path
Method (CPM)
• E I Du Pont de Nemours & Co. (1957)
for construction of new chemical plant
and maintenance shut-down
• Deterministic task times
• Activity-on-node network
construction
• Repetitive nature of jobs
CPM

 Path
 A connected sequence of activities leading from the starting
event to the ending event
 Critical Path
 The longest path (time); determines the project duration
 Critical Activities
 All of the activities that make up the critical path
CPM calculation

 Useful at many stages of project management
 Mathematically simple
 Give critical path and slack time
 Provide project documentation
 Useful in monitoring costs
Benefits of CPM/PERT

 Clearly defined, independent and stable activities
 Specified precedence relationships
 Over emphasis on critical paths
 Deterministic CPM model
 Activity time estimates are subjective and depend on judgment
 PERT assumes a beta distribution for these time estimates, but
the actual distribution may be different
 PERT consistently underestimates the expected project
completion time due to alternate paths becoming critical
Limitations to CPM/PERT

Activity
Predecessor
activity
A none
B none
C A
D A
E B
F C
G D & E
1
3
2
5
4
6
A
B
C
D
E
F
G
Example

Activity
Predecessor
activity
A none
B A
C A
D B
E C
F D ,E
Example

Project Crashing
 Crashing
 reducing project time by expending additional resources
 Crash time
 an amount of time an activity is reduced
 Crash cost
 cost of reducing activity time
 Goal
 reduce project duration at minimum cost

Time-Cost Relationship
 Crashing costs increase as project duration decreases
 Indirect costs increase as project duration increases
 Reduce project length as long as crashing costs are less than
indirect costs
Time-Cost Tradeoff
time
Direct cost
Indirect
cost
Total project cost

Activity
Normal
(Wks)
Crush
(Wks)
Cost
Slope
(K$)
Tn Cn Tc Cc
A 9 10 6 16 2
B 8 9 5 18 3
C 5 7 4 8 1
D 8 9 6 19 5
E 7 7 3 15 2
F 5 5 5 5 -
G 5 8 2 23 5
Crashing Example

A
D
G
F
C
B
E
9
8
8
7
5
5
5
0
22
149
17
8
2
2
10
0
9 1
7
17
Critical Path A-D-G=22wks

The Resource Problem
 Resources and Priorities
 Project network times are not a schedule until resources have
been assigned.
 The implicit assumption is that resources will be available in the
required amounts when needed.
 Adding new projects requires making realistic judgments of resource
availability and project durations.
 Resource-Constrained Scheduling
 Resource leveling (or smoothing) involves attempting to even
out demands on resources by using slack (delaying noncritical
activities) to manage resource utilization.

 People
 Materials
 Equipment
 Working Capital
Kinds of resource

 A network analysis technique used in project management.
 It allows probabilistic treatment of both network logic and
activity duration estimated.
 The technique was first described in 1966 by Dr. Alan B.
Pritsker of Purdue University.
 Compared to other techniques, GERT is an only rarely used
scheduling technique.
GERT

Contd..
 Utilizes probabilistic and branching nodes
 It represents the node will be reached if any m of its p
immediate predecessors are completed.
m
n
p

Contd..
 It represents a probabilistic output where any of q
outputs are possible
 Each branch has an assigned probability
 When no probability is given, the probability is assumed
to be one for each branch.
1
2
q

Pert, cpm & gert

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