Network Analysis and Duration Estimating

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Network Analysis and Duration Estimating

• Kathy S. Schwaig

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A Roadmap of the Project Planning Process

• Develop a business case
• Select a project
• Develop project charter
• Establish work breakdown structure
• Analyze sequencing relationships
• Estimate normal activity durations
• Perform network calculations
• Validate/revise initial schedule
• Perform time-cost tradeoff analysis
• Load resources to activities
• Resolve any resource/workload imbalances
• Develop budget and cash flow plan based on
  analysis of direct and indirect costs

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Network-based tools to model sequencing
relationships

• Critical path method (CPM) ?Our Focus Here
• Developed by Dupont and Remington Rand in the
  late 1950s for managing plant maintenance
  projects
• Uses one duration estimate for each activity
• Provides basic framework for project planning and
  control
• Program evaluation and review technique (PERT)
• Developed in conjunction with Lockheeds
  development of the Polaris Missile in the late
  1950s
• Requires three duration estimates for each
  activity (optimistic, most likely, pessimistic)
• Allows for crude risk assessment on overall
  project duration

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Analyzing Sequencing Relationships

• The work breakdown structure gives you the tasks
  or activities that have to be accomplished
• The next step is to determine the sequencing of
  those activities
• The sequence of activities can be represented in
  the form of a network

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Activities-on-nodes (AON) project network

• In an AON project network
• the activities are the nodes in the network
• the precedence relationships are shown by arrows
• An AON project network should have one starting
  node and one ending node
• The project network represents a model of the
  project and shows the relationships among
  activities
• Example

B
E
A
D
C
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Developing a project network adding activities

• In developing a project network, you may identify
  additional activities
• To determine which activities should be added to
  the network, it is helpful to ask the following
  question
• Given where we are in the project, what
  activity(s) can we perform next?

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Developing a project network determining
relationships

• In developing a project network, you will need to
  determine the relationships among activities
• To determine how to connect an activity into the
  network, it is helpful to ask the following
  question
• Which activity(s) would have to be finished
  before this activity could start?

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Precedence Relationships

• Finish-to-Start (FS)
• Start-to-Start (SS)
• Finish-to-Finish (FF)

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Building a Project NetworkAn Example

• Suppose our project charter is to bake a birthday
  cake from scratch with homemade chocolate
  frosting
• Assumptions and constraints
• All required ingredients and utensils are on-hand
• Recipe exists and must be read first before any
  other activity can begin
• Cleanup at end of project (hint this is your
  last activity)
• Draw AON project network using finish-to-start
  (FS) precedence relationships

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Exercise Draw an AON network for this project

• Activities A and B have no predecessors
• Activity C can start when A is completed
• When both A and B are finished, activity D can
  start
• Activity E is dependent only on the completion of
  B
• Activity F can start when C and E are completed
• When D is finished, activity G can start
• Activity H cannot start until both E and G are
  finished

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Guidelines for Developing Project Network Diagrams

• Make sure that your precedence relationships
  reflect technical reasons for task A preceding
  task B
• Label your nodes with short activity descriptions
  (not codes)
• AON networks should have one starting node and
  one ending node

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Guidelines for Developing Project Network Diagrams

• Use FS precedence relationships wherever possible
• Each precedence arrow should connect two
  activities
• Do not put any feedback loops in your network
  diagram
• Limit your AON project network to no more than
  about 50 nodes

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Estimating Activity Durations

• Activity duration is the amount of time between
  the start and completion of the activity (not
  equal to staff hours)
• days is the typical unit of time
• normal point is the duration associated with
  the most efficient use of resources (i.e. lowest
  cost)
• crash point is the shortest amount of time in
  which the activity can be done successfully

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Guidelines for Estimating Activity Durations

• Define activity scope and content
• Determine most cost efficient technological
  approach
• Determine which staff members will be assigned
• Estimate staff hours to complete activity
• Estimate average availability of assigned staff
  members
• Duration days staff hours required/available
  staff hours per day
• Selectively adjust durations of activities that
  are subject to common problems

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Guidelines for Estimating Activity Durations

• Dont confuse duration (days) with resource usage
  (staff hours or days)
• Allow for less than full time resource
  availability
• Base your estimates on clearly defined activity
  scope
• Allow for delays caused by common problems
• Dont pad or low ball estimates
• No duration estimates should be longer than 2
  weeks (80 hour rule)
• Seek commitment to duration estimates from the
  people who are doing the work

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Labeling of Network Nodes

• Each node is labeled with certain information
• ACTname of activity
• Dduration of activity
• EPSearliest possible starting time
• EPCearliest possible completion time
• LASlatest allowable starting time
• LAClatest allowable completion time
• TStotal slack

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Total Slack vs. Free Slack

• Total (Path) Slack (sometimes called float)
• Amount of time by which the activity can be
  delayed beyond its earliest possible completion
  time (EPC) without delaying the project beyond
  its latest allowable completion time (LAC)
• Total Slack, TS LAC - EPC
• Free (Activity) Slack
• Amount of time by which the activity can be
  delayed beyond its earliest possible completion
  time (EPC) without delaying the start of any
  other activity beyond its earliest possible
  starting time (EPS)

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Network Calculations

• Forward pass calculations (EPS EPC)
• The EPS for the first activity in the project
  network is usually set at zero
• The EPS for any other activity is the largest (or
  latest) of the EPC values for all immediately
  preceding connected activities
• The EPC for any activity is calculated as
  follows EPCEPSD

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Network Calculations

• Backward pass calculations (LAS LAC)
• The LAC for the last activity in the project
  network is usually set equal to the EPC for that
  activity (or to some specified completion
  deadline)
• The LAC for any other activity is the smallest
  (or earliest) of the LAS values for all
  immediately following connected activities
• The LAS for any activity is computed as follows
• LASLAC-D
• When calculations are complete, LAS-EPS for the
  first activity in the network should equal
  LAC-EPC for the last activity in the network

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Network Calculation Example
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Finding the Critical Path(s)

• A critical path is a connected series of
  activities whose combined duration is the longest
  of any path through the project network
• Critical path can be found by
• Tracing EPS
• Go to last activity
• Circle earliest possible start (EPS)
• Find which predecessor activity node is supplying
  that EPS
• Repeat until you get back to the start of the AON
  network

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Why the Critical Path Matters

• The critical path determines project duration
  (because its the longest path through the
  network)
• A project can have more than one critical path
• To shorten project, it is necessary to shorten
  the durations of all critical paths
• Any delay along any critical path will delay
  project completion
• Activities on the critical path have the lowest
  total slack value in the network

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Crashing the Network to Shorten the Project
Duration

• Focus on activities that are on the critical path
• Look for activities with relatively long
  durations
• Look for activities that are on multiple critical
  paths