Network Scheduling and Precedence Diagramming Method PDM MT 246 Module 10

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Network Scheduling and Precedence Diagramming
Method (PDM) - MT 246 - Module 10
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Topics Covered Previously

• Scheduling
• Events/Activities
• Types of Schedules
• Gantt or Bar Charts
• Precedence Diagrams/Networks
• Task Relationships
• Percent Complete
• Resource Loading and Curves

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Precedence Diagrams or Networks

• Shows tasks and their relationships
• Exposes tasks that must be completed before
  others
• Called a precedence diagram
• Arrows show how tasks are ordered and flow of time

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First and Last Events

• First event has no predecessors
• Last event has no successors
• All networks have a first and last event

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Types of Network Diagrams

• Activity on Node (AON)
• Used in CPM
• Activity on Arc (AOA)
• Used in PERT

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Activity on Node (AON) Diagrams

• Bubble Chart
• Events represented by lines
• Delimited by Bubbles (Activities)
• Activities are in bubbles

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AON Chart based on Activities and Predecessors
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Activity on Arc (AOA) Diagrams

• Events are in bubbles
• Activities represented by lines or arcs
• Delimited by Bubbles (Events)
• Requires Dummy Activities to illustrate
  precedence

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Activity on Arrow (Arc) Diagrams
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Dummy Activities

• Required due to AOA rule that each task is
  represented by one Arc, which connects two
  events
• Parallel tasks have a different duration
• If terminated on one event (instant in time),
  implication is that they both have the same
  duration
• Not generally the case
• Cannot change the schedule if one event slips
  without revising the diagram

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Dummy Activity Design Rules

• Add Dummy Activity wherever necessary when first
  generating the network
• Remove non-essential dummies
• Overriding Rule
• Do not remove dummy activity when the result
  creates two or more activities between a
  start/finish node pair

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AOA Diagram based on Activities and Predecessors
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Network Design Rules

• Length of lines and/or placement of bubbles have
  no significance regarding task duration
• Gantt chart task duration indicated by length of
  bar
• Level of detail must show all schedule
  constraints
• Plan for schedule changes during project
• Allow for workarounds
• Show all significant events/milestones
• Network must/should correlate to WBS
• Clean schedule interfaces

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Network Design Rules (Continued)

• Earliest time is on the left, latest time is on
  the right
• Always use a single start event
• Contract Award
• No predecessors
• Always use a single completion event
• System Delivery
• No successors

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AON vs. AOA

• AON networks do not use dummy events
• Simpler and easier to generate than AOA
• Popular in Construction Industry
• AOA method places emphasis on Events
• Developed before AON
• Better for PERT charts
• AOA line segments imply flow of time
• Look similar to Gantt Charts
• Most Software packages create both

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Schedule Network Methodology

• Tabulate Tasks (Activities)
• Determine Task Duration
• Determine Immediate Predecessors
• Assign Start Event Date and Time

We are only interested in Process
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Start Event

• Establish Start Date
• Calculate all other event dates using Schedule
  Logic and Task Duration
• Schedule network determines dates, not the other
  way around
• If finish date is incompatible with project
  goals, you must adjust the schedule assumptions
• Logic (Workarounds)
• Task Duration (More People or Overtime)

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LOGON Task Table
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LOGON AON Diagram
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Incorrect AOA Example
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Key Features of Schedule Networks

• Critical Path
• Early Times
• Start
• Finish
• Late Time
• Start
• Finish
• Total Slack
• Free Slack

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Critical Path

• Concept of Path Length through network
• Calculate for all possible paths by traversing
  Network from left to right
• Longest path length from start event to finish
  event is critical path
• Activities on the critical path cause a
  day-for-day slip in the completion event
• After calculating the critical path, look for
  things that can be done in parallel
• Shortens critical path

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LOGON Critical Path Determination
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Early Expected Time of an Event

• Designated as TE
• Calculation of Early Expected Time is part of
  schedule analysis
• Events which are not on the critical path can be
  started early
• By definition, events on the critical path cannot
  be started early
• Can have more than one critical path, on that
  path events cannot be started early

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Early Expected Time of an Event (Continued)

• Calculated by taking the sum of all task
  durations on the longest path leading to the
  event
• Traverse network from left to right

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Early Expected Time Determination
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Latest Allowable Time of an Event

• Designated as TL
• Latest time to which an event can be slipped
  without affecting succeeding events
• Calculated by taking the sum of all task
  durations on the longest backward path from the
  finish date to the event of interest
• Traverse network from right to left

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Latest Allowable Time Determination
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Total Slack

• Slack is the range of allowable time between when
  a task can be started, and when it must be
  started
• Once slack is used up, the finish date of the
  project is affected
• The task of interest is now on the critical path
• Total slack of an activity is the amount of slack
  available to all activities on a given subpath of
  a network
• Total slack of activities on the critical path is
  zero

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Total Slack (Continued)

• Total Slack of an activity (task) is calculated
  as follows

Total Slack LS - ES
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Free Slack

• Activities not on the critical path can be
  delayed without affecting the start time of
  succeeding tasks
• Free slack of an activity is the amount of time
  that the activity can slip without affecting its
  successors
• Assumes that the TE of all preceding tasks has
  been met
• Free Slack of an activity is calculated as
  follows

Free Slack ES (earliest successor) - EF
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An Example of Early and Late Times for AON
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Precedence Diagramming Method (PDM)

• Allows relationships between predecessor tasks
  which are partially completed
• Start-to-Start,SS
• Finish-to-Finish, FF
• Start-to-Finish, SF
• Finish-to-Start, FS
• Multiple PDM Relationships

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Start-to-Start,SS
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Finish-to-Finish, FF
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Start-to-Finish, SF
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Finish-to-Start, FS
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Multiple PDM Relationships
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Pros and Cons of PDM

• Greater Flexibility than AOA and AON
• Critical Path and Slack Times are not simple
• Complex Relationships give Counterintuitive
  Results
• Requires more care
• Included in modern SW packages

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Criticisms of Network Methods

• Unrealistic Results
• Assume Project can be completely defined (not
  true) they evolve
• No Clean line between activities (precedence
  helps)
• Precedence relationships are not fixed (do-loop
  may be necessary, i.e., test)
• They do produce the best schedule possible

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Summary

• Introduced network methods and PDM for scheduling
• Networks display the connections between project
  activities and impact on each other
• Determine critical activities and slack times
• PDM reflects the realities of projects work

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Reading and Review AssignmentMidterm Exam
Information

• Read Nicholas Chapter 7
• Answer the following Review Questions and
  Problems 7-1,2,4,5, 10a, 13b,15a,17
• Midterm Exam (take-home)
• Class handout of MT exam on 22 February
• Return on 27 February to my office (no class on
  27 February)

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Next Lecture

• PERT
• CPM
• Resource Allocation
• GERT