Network Analysis and Duration Estimating

Kathy S. Schwaig

A Roadmap of the Project Planning ProcessDevelop a business caseSelect a projectDevelop project charterEstablish work breakdown structureAnalyze sequencing relationships Estimate “normal” activity durationsPerform network calculationsValidate/revise initial schedulePerform time-cost tradeoff analysisLoad resources to activitiesResolve any resource/workload imbalancesDevelop budget and cash flow plan based on analysis of direct and indirect costs

Network-based tools to model sequencing relationships

Critical path method (CPM) Our Focus HereDeveloped by Dupont and Remington Rand in the late 1950s for managing plant maintenance projectsUses one duration estimate for each activityProvides 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 1950sRequires three duration estimates for each activity (optimistic, most likely, pessimistic)Allows for crude risk assessment on overall project duration

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

Activities-on-nodes (AON) project network

In an AON project network:the activities are the nodes in the networkthe precedence relationships are shown by arrows

An AON project network should have one starting node and one ending nodeThe project network represents a model of the project and shows the relationships among activitiesExample:

EA D

C

B

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?

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?

Precedence Relationships

Finish-to-Start (FS)

Start-to-Start (SS)

Finish-to-Finish (FF)

A B

S F FS

A

S F

B

FS

B

FSA

S F

Building a Project Network:An Example

Suppose our project charter is to bake a birthday cake from scratch with homemade chocolate frostingAssumptions and constraints:

All required ingredients and utensils are on-handRecipe 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

Exercise: Draw an AON network for this project

Activities A and B have no predecessorsActivity C can start when A is completedWhen both A and B are finished, activity D can startActivity E is dependent only on the completion of BActivity F can start when C and E are completedWhen D is finished, activity G can start Activity H cannot start until both E and G are finished

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

Guidelines for Developing Project Network Diagrams

Use FS precedence relationships wherever possibleEach precedence arrow should connect two activitiesDo not put any “feedback loops” in your network diagramLimit your AON project network to no more than about 50 nodes

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

Guidelines for Estimating Activity Durations

Define activity scope and contentDetermine most cost efficient technological approachDetermine which staff members will be assignedEstimate staff hours to complete activityEstimate average availability of assigned staff membersDuration days = staff hours required/available staff hours per daySelectively adjust durations of activities that are subject to common problems

Guidelines for Estimating Activity Durations

Don’t confuse duration (days) with resource usage (staff hours or days)Allow for less than full time resource availabilityBase your estimates on clearly defined activity scopeAllow for delays caused by common problemsDon’t “pad” or “low ball” estimatesNo duration estimates should be longer than 2 weeks (80 hour rule)Seek commitment to duration estimates from the people who are doing the work

Labeling of Network Nodes

ACT

D TS

EPS EPC

LAS LAC

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

Total Slack vs. Free SlackTotal (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) SlackAmount 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)

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

Network CalculationsBackward 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 activitiesThe 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

Network Calculation Example

A

GD

FC

B E

H

END2

8 4

2

06 6

4 10

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

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 pathTo shorten project, it is necessary to shorten the durations of all critical paths Any delay along any critical path will delay project completionActivities on the critical path have the lowest total slack value in the network

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