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Network Scheduling and Precedence Diagramming Method PDM MT 246 Module 10

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First event has no predecessors. Last event has no successors ... AON Chart based on Activities and Predecessors. Activity on Arc (AOA) Diagrams ... – PowerPoint PPT presentation

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Title: Network Scheduling and Precedence Diagramming Method PDM MT 246 Module 10


1
Network Scheduling and Precedence Diagramming
Method (PDM) - MT 246 - Module 10
2
Topics Covered Previously
  • Scheduling
  • Events/Activities
  • Types of Schedules
  • Gantt or Bar Charts
  • Precedence Diagrams/Networks
  • Task Relationships
  • Percent Complete
  • Resource Loading and Curves

3
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

4
First and Last Events
  • First event has no predecessors
  • Last event has no successors
  • All networks have a first and last event

5
Types of Network Diagrams
  • Activity on Node (AON)
  • Used in CPM
  • Activity on Arc (AOA)
  • Used in PERT

6
Activity on Node (AON) Diagrams
  • Bubble Chart
  • Events represented by lines
  • Delimited by Bubbles (Activities)
  • Activities are in bubbles

7
AON Chart based on Activities and Predecessors
8
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

9
Activity on Arrow (Arc) Diagrams
10
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

11
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

12
AOA Diagram based on Activities and Predecessors
13
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

14
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

15
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

16
Schedule Network Methodology
  • Tabulate Tasks (Activities)
  • Determine Task Duration
  • Determine Immediate Predecessors
  • Assign Start Event Date and Time

We are only interested in Process
17
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)

18
LOGON Task Table
19
LOGON AON Diagram
20
Incorrect AOA Example
21
Key Features of Schedule Networks
  • Critical Path
  • Early Times
  • Start
  • Finish
  • Late Time
  • Start
  • Finish
  • Total Slack
  • Free Slack

22
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

23
LOGON Critical Path Determination
24
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

25
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

26
Early Expected Time Determination
27
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

28
Latest Allowable Time Determination
29
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

30
Total Slack (Continued)
  • Total Slack of an activity (task) is calculated
    as follows

Total Slack LS - ES
31
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
32
An Example of Early and Late Times for AON
33
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

34
Start-to-Start,SS
35
Finish-to-Finish, FF
36
Start-to-Finish, SF
37
Finish-to-Start, FS
38
Multiple PDM Relationships
39
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

40
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

41
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

42
Reading and Review Assignment Midterm 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)

43
Next Lecture
  • PERT
  • CPM
  • Resource Allocation
  • GERT
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