Title: Network Analysis and Duration Estimating
1Network Analysis and Duration Estimating
2A 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
3Network-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
4Analyzing 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
5Activities-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
6Developing 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?
7Developing 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?
8Precedence Relationships
- Finish-to-Start (FS)
- Start-to-Start (SS)
- Finish-to-Finish (FF)
9Building 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
11Guidelines 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
12Guidelines 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
13Estimating 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
14Guidelines 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
15Guidelines 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
16Labeling 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
17Total 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)
18Network 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
19Network 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
20Network Calculation Example
21Finding 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
22Why 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
23Crashing 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