Precedence Diagram Methods

8/3/2019 Precedence Diagram Methods

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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 othersCalled 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 withoutrevising 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 eventContract Award

No predecessors

Always use a single completion eventSystem 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 timeLook 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 traversingNetwork from left to right

Longest path length from start event to finishevent is critical path

Activities on the critical path cause a day-for-dayslip in the completion event

After calculating the critical path, look for thingsthat 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 partof schedule analysis

Events which are not on the critical path canbe started early

By definition, events on the critical path cannot

be started earlyCan 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

R di d R i A i


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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)


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Next Lecture

PERT

CPM

Resource Allocation GERT

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Precedence Diagram Methods

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