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NETWORK SCHEDULING TECHNIQUES

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Network DiagramsPMI defines the scheduling process as:

“the identification of the project objectives and the ordered activity necessary to complete the project including the identification of resource types and quantities required.”

Project scheduling defines the network logic for all activities that must either precede or succeed other tasks from the beginning of the project until its completion.

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Network DiagramsProvide a basis for planning and how to use the resourcesIdentify the critical path and project completion timeIdentify where slacks (float) areReveal interdependencies of activitiesAid in risk analysis (what-if analysis)

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Network Diagrams

Show interdependence

Facilitate communication

Help schedule resources

Identify critical

activities

Determine project completion

Show start & finish dates

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Network Scheduling TechniquesNetwork scheduling techniques provide a logical process to consider the order in which the project activities should occur.The primary methods for developing project activity networks are:

Program Evaluation and Review Technique (PERT)Critical Path Method (CPM) – Also called Arrow Diagram Method (ADM)Precedence Diagram Method (PDM)

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Network Scheduling TechniquesThere are two ways to show the network:

Activity-On-Node (AON) – nodes represent the activitiesActivity-On-Arch (AOA) – archs represent the activities

AON is easier, and it used in commercial software.

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AOA vs. AON

activities on arc

C

ED

B F

E

C

DB F

activities on node

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PERT/CPM

PERT was developed in the late 1950s in collaboration between the US Navy, Booz-Allen Hamilton and Lockeed Corporation for the creation of the Polaris missile program.CPM was developed at the same time by DuPont.Over the years the differences between PERT and CPM have blurred, so it is common to refer these techniques as just PERT/CPM.

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Precedence Diagramming Method (PDM)

PERT/CPM networks do not allow for leads and lags between two activities; i.e. a preceding activity must be completely finished before the start of the successor activity.Precedence Diagramming Method (PDM) allows these leads and lags.Most project management software systems use PDM and show interrelationships on bar charts.

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Precedence Networkin a Gannt Chart

TASKS 1 2 3 4 5

4

MONTHS AFTER GO-AHEAD

3

2

1

5

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Network Development Rules All activities must be linked to each otherNetwork diagrams flow from left to rightAn activity cannot begin until all preceding connected activities have been completedEach activity should have a unique identifier (number, letter, code, etc.)Looping is not permittedIt is common to start from a single beginning and finish on a single ending node

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Steps in Creating the NetworkDefine the project and all of its significant activitiesDevelop the relationship among activitiesDecide which activities must precede othersDraw the network connecting all of the activitiesCompute the longest path which is the critical pathCalculate activity slacks (float)Use the network to help plan, schedule, and control the project

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Node Labels Nodes representing activities should be labeled with the following information:

IdentifierDescriptionDurationEarly Start TimeEarly Finish TimeLate Start TimeLate Finish TimeFloat

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Node Labels

Early Start

Activity Float Activity Descriptor

Late Start

ID Number

Activity Duration

Late Finish

Early Finish

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Node Labels Early Start (ES) – Earliest possible date an activity can start based on the network logic and any schedule constraints.Early Finish (EF) = ES + DurLate Start (LS) – Latest possible date an activity may begin without delaying a specified milestone (usually project finish date).Late Finish (LF) = LS + Dur

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Project Scheduling Terms

• Successors

• Predecessors

• Network diagram

• Serial activities

• Concurrent activities

ED

C

B

A F

• Merge activities

• Burst activities

• Node

• Path

• Critical Path

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Project Scheduling ActivitiesSerial activities flow from one to the nextConcurrent activities are accomplished at the same timeMerge activities have two or more immediate predecessorBurst activities have two or more successor activities

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Serial Activities

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Parallel Activities

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Merge Activities

Activity A

Activity B

Activity C

Activity D

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Burst Activities

Activity C

Activity B

Activity A

Activity D

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Example

Activity Description Predecessors DurationA Contract signing None 5B Questionnaire design A 5C Target market ID A 6D Survey sample B, C 13E Develop presentation B 6F Analyze results D 4G Demographic analysis C 9H Presentation to client E, F, G 2

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Example

A Contract

5

C Market ID

6

B Design

5

G Demog.

9

EDev. Present.

6

D Survey

13

F Analysis

4

H Present

2

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ExamplePath One: A-B-E-H = 18 weeksPath Two: A-B-D-F-H = 29 weeksPath Three: A-C-D-F-H = 30 weeksPath Four: A-C-G-H = 22 weeks

Path three is the critical path

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Forward PassForward pass determines the earliest times (ES) each activity can begin and the earliest it can be completed (EF).There are three steps for applying the forward pass:

Add all activity times along each path as we move through the network (ES + Dur = EF)Carry the EF time to the activity nodes immediately succeeding the recently completed node. That EF becomes the ES of the next node, unless the succeeding node is a merge pointAt a merge point, the largest preceding EF becomes the ES for that node (because the earliest the successor can begin is when all preceding activities have been completed)

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Forward Pass

0 A 5Contract

5

5 B 10Design

5

5 C 11Market ID

6

11 D 24Survey

13

11 G 20Demog.

9

24 F 28Analysis

4

10 E 16Dev. Present

6

28 H 30Present

2

Activity D is a merge point for B and CActivity H is a merge point for E, F, and G

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Backward PassThe goal of the backward pass is to determine each activity's Late Start (LS) and Late Finish (LF) times.There are three steps for applying the backward pass:

Subtract activity times along each path through the network (LF – Dur = LS).Carry back the LS time to the activity nodes immediately preceding the successor node. That LS becomes the LF of the next node, unless the preceding node is a burst point.In the case of a burst point, the smallest succeeding LS becomes the LF for that node (because the latest the predecessor can finish is when any one of the successor activities should start)

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Backward Pass

0 A 5Contract

0 5 5

5 B 10Design

6 5 11

5 C 11Market ID

5 6 11

11 D 24Survey

11 13 24

11 G 20Demograph.

19 9 28

24 F 28Analysis

24 4 28

10 E 16Dev. Present 22 6 28

28 H 30Presentation

28 2 30

Activities A, B, and C are burst points

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Slack Time (Float)Since there exists only one path through the network that is the longest, the other paths must either be equal or shorter.Therefore, there are activities that can be completed before the time when they are actually needed.The time between the scheduled completion date and the required date to meet critical path is referred as the slack time.The activities on the critical path have zero slack time.

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Slack Time (Float) The use of slack time provides better resource scheduling.It is also used as warning sign i.e. if available slack begins to decrease then activity is taking longer than anticipated.Slack time is equal to:

LS – ES or LF – EFActivities on the critical path have 0 slack; i.e. any delay in these activities will delay the project completion.

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Complete Activity Network

0 A 50 Contract

0 5 5

5 B 101 Design6 5 11

5 C 110 Market ID

5 6 11

11 D 240 Survey

11 13 24

11 G 208 Demograph.19 9 28

24 F 280 Analysis24 4 28

10 E 1612 Dev. Present

22 6 28

28 H 30 0 Presentation

28 2 30

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Reducing the Critical Path Eliminate tasks on the Critical Path Convert serial paths to parallel when possible Overlap sequential tasks Shorten the duration on critical path tasks Shorten

early taskslongest taskseasiest taskstasks that cost the least to speed up

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Lag Lag is the time between Early Start or Early Finish of one activity and Early Start and Early Finish on another activity.For example, in a Finish-to-Start dependency with a 10-day lag, the successor activity cannot start until 10 days after the predecessor activity has finished.Lags are not the same as slacks. Lags are between activities whereas slacks are within activities.

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Finish to Start Lag• Most common type of sequencing

• Shown on the line joining the modes– Added during forward pass– Subtracted during backward pass

0 A 6Spec Design

6

6 B 11Design Check

5

15 C 22Blueprinting

7

Lag 4

This lag is not

the same as

activity slack

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LeadLead allows an acceleration of the successor activity. We can expedite the schedule by not waiting a preceding activity to be completely finished before starting its successor. For example, in a Finish-to-Start dependency with a 10-day lead, the successor activity can start 10 days before the predecessor activity has finished.

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Laddering Activities

Project ABC can be completed more efficiently if subtasks are used (Fast Tracking)

A(3) B(6) C(9) ABC=18 days

Laddered ABC=12 days

A1(1) A2(1) A3(1)

B1(2) B2(2) B3(2)

C1(3) C2(3) C3(3)

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Hammock Activities

Used as summaries for subsets of activities

0 A 5

0 5 5

5 B15

5 1015

15 C 18

15 3 18

0 Hammock 18

0 18 18

Useful with a complex project or one that has

a shared budget