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Network Analysis and Duration Estimating

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Title: Network Analysis and Duration Estimating


1
Network Analysis and Duration Estimating
  • Kathy S. Schwaig

2
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

3
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

4
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

5
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
6
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?

7
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?

8
Precedence Relationships
  • Finish-to-Start (FS)
  • Start-to-Start (SS)
  • Finish-to-Finish (FF)

9
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

10
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

11
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

12
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

13
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

14
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

15
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

16
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

17
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)

18
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

19
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

20
Network Calculation Example
21
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

22
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

23
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
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