Network Scheduling, Planning & Controlling

Techniques

By Pranava Krishna

IntroductionSchedule converts action plan into operating time

tableBasis for monitoring and controlling projectScheduling more important in projects than in

production, because unique natureSometimes customer specified/approved

requirement.Based on Work Breakdown Structure (WBS)

Graphical portrayal of activities and eventShows dependency relationships between

tasks/activities in a projectClearly 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

GERT

PERT

History Of PERT/CPMDeveloped 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 deliverablesPlanning resource requirementsControlling resource allocationInternal program reviewExternal program reviewPerformance evaluationUniform 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/CPMPrediction of deliverablesPlanning resource requirementsControlling resource allocationInternal program reviewExternal program reviewPerformance evaluationUniform wide acceptance

PERT

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 wellmost likely time (m ) - the consensus best estimate

of the activity’s durationoptimistic time (b) - the time the activity would

take if things did go wellMean (expected time):te = a + 4m + b

6

Variance: V =b- a

6

2

PERT

Use of PERTIn construction activities Transportation activitiesIn oil refineriesComputer system-For manufacturing electric generator machinesMedical and surgical sectorLibrary activities

Importance of PERT SystemReduction in costSaving of timeDetermination of activitiesElimination of risk in complex activities –FlexibilityEvaluation of alternatives-Useful in effective control-Useful in decision makingUseful 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

CPM

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

PathA connected sequence of activities leading from

the starting event to the ending eventCritical Path

The longest path (time); determines the project duration

Critical ActivitiesAll of the activities that make up the critical

path

CPM calculation

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

Useful at many stages of project managementMathematically simpleGive critical path and slack timeProvide project documentationUseful in monitoring costs

Benefits of CPM/PERT

Clearly defined, independent and stable activitiesSpecified precedence relationshipsOver emphasis on critical pathsDeterministic CPM modelActivity time estimates are subjective and depend

on judgmentPERT 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 noneB noneC AD AE BF CG D & E

1

3

2

5

4

6

A

B

C

D

E

F

G

Example

Activity

Predecessor activity

A noneB AC AD BE CF D ,E

Example

1 2

3

4

5 6A

B

C

D

E

F

Project CrashingCrashing

reducing project time by expending additional resources

Crash timean amount of time an activity is reduced

Crash costcost of reducing activity time

Goalreduce 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 costsTime-Cost Tradeoff

cost

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 2B 8 9 5 18 3C 5 7 4 8 1D 8 9 6 19 5E 7 7 3 15 2F 5 5 5 5 -G 5 8 2 23 5

Crashing Example

AD

G

FC

B E

98

87

55

5

0 22

149

178

22

10

0

9 17

17

Critical Path A-D-G=22wks

The Resource ProblemResources 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.

PeopleMaterialsEquipmentWorking Capital

Kinds of resource

GERT

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 nodesIt 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 possibleEach branch has an assigned probabilityWhen no probability is given, the probability is

assumed to be one for each branch.

1

2

q

 

Example

Thank you