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Network Analysis and Duration Estimating Kathy S. Schwaig.

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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 Lockheed’s 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: EAD C B

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) AB SFFS A SF B FS B FS A SF

9 Building a Project Network: An 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 Don’t 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 Don’t “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 ACT DTS EPSEPC LASLAC Each node is labeled with certain information ACT=name of activity D=duration of activity EPS=earliest possible starting time EPC=earliest possible completion time LAS=latest allowable starting time LAC=latest allowable completion time TS=total 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: EPC=EPS+D

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: LAS=LAC-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 A GD FC BE H END

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 it’s 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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