NETWORK SCHEDULING TECHNIQUES

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NETWORK SCHEDULING TECHNIQUES
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Network Diagrams

• PMI 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 Diagrams

• Provide a basis for planning and how to use the
  resources
• Identify the critical path and project completion
  time
• Identify where slacks (float) are
• Reveal interdependencies of activities
• Aid in risk analysis (what-if analysis)

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Network Diagrams
Help schedule resources
Show interdependence
Show start finish dates
Facilitate communication
Identify critical activities
Determine project completion
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Network Scheduling Techniques

• Network 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 Techniques

• There are two ways to show the network
• Activity-On-Node (AON) nodes represent the
  activities
• Activity-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

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 the 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
MONTHS AFTER GO-AHEAD
TASKS
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1
2
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5
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Network Development Rules

• All activities must be linked to each other
• Network diagrams flow from left to right
• An activity cannot begin until all preceding
  connected activities have been completed
• Each activity should have a unique identifier
  (number, letter, code, etc.)
• Looping is not permitted
• It is common to start from a single beginning and
  finish on a single ending node

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Steps in Creating the Network

• Define the project and all of its significant
  activities
• Develop the relationship among activities
• Decide which activities must precede others
• Draw the network connecting all of the activities
• Compute the longest path which is the critical
  path
• Calculate 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
• Identifier
• Description
• Duration
• Early Start Time
• Early Finish Time
• Late Start Time
• Late Finish Time
• Float

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Node Labels
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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 Dur
• Late 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

• Merge activities
• Burst activities
• Node
• Path
• Critical Path

• Successors
• Predecessors
• Network diagram
• Serial activities
• Concurrent activities

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Project Scheduling Activities

• Serial activities flow from one to the next
• Concurrent activities are accomplished at the
  same time
• Merge activities have two or more immediate
  predecessor
• Burst 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 D
Activity C
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Burst Activities
Activity B
Activity C
Activity A
Activity D

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Example
Activity Description Predecessors Duration A Con
tract signing None 5 B Questionnaire
design A 5 C Target market ID A 6 D Survey
sample B, C 13 E Develop presentation B 6 F A
nalyze results D 4 G Demographic
analysis C 9 H Presentation to client E, F, G 2
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Example
E Dev. Present. 6
B Design 5
A Contract 5
F Analysis 4
H Present 2
D Survey 13
C Market ID 6
G Demog. 9
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Example

• Path One A-B-E-H 18 weeks
• Path Two A-B-D-F-H 29 weeks
• Path Three A-C-D-F-H 30 weeks
• Path Four A-C-G-H 22 weeks
• Path three is the critical path

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Forward Pass

• Forward 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 point
• At 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
5 B 10 Design 5
10 E 16 Dev. Present 6
11 D 24 Survey 13
24 F 28 Analysis 4
28 H 30 Present 2
0 A 5 Contract 5
5 C 11 Market ID 6
11 G 20 Demog. 9
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 Pass

• The 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
10 E 16 Dev. Present 22 6 28
5 B 10 Design 6 5 11
28 H 30 Presentation 28 2 30
0 A 5 Contract 0 5 5
24 F 28 Analysis 24 4 28
11 D 24 Survey 11 13 24
5 C 11 Market ID 5 6 11
11 G 20 Demograph. 19 9 28
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 EF
• Activities 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
5 B 10 1 Design 6 5
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10 E 16 12 Dev. Present 22 6 28
0 A 5 0 Contract 0 5
5
11 D 24 0 Survey 11 13 24
24 F 28 0 Analysis 24 4 28
28 H 30 0 Presentation 28 2
30
11 G 20 8 Demograph. 19 9 28
5 C 11 0 Market ID 5 6 11
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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 tasks
• longest tasks
• easiest tasks
• tasks 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

This lag is not the same as activity slack
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Lead

• Lead 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)

ABC18 days
Laddered ABC12 days
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Hammock Activities

• Used as summaries for subsets of activities

Useful with a complex project or one that has a
shared budget