Presentation on theme: "Project Scheduling: Networks, Duration Estimation, and Critical Path"— Presentation transcript:
1Project Scheduling: Networks, Duration Estimation, and Critical Path Chapter 9
2Learning Goals Understand and apply key scheduling terminology. Apply the logic used to create activity networks, including predecessor and successor tasks.Develop an activity network using Activity-on-Node (AON) techniques.Perform activity duration estimation based on the use of probabilistic estimating techniques.Construct the critical path for a project schedule network using forward and backward passes.Identify activity float and the manner in which it is determined.Understand the steps that can be employed to reduce the critical path.
3Project SchedulingProject scheduling requires us to follow some carefully laid-out steps, in order, for the schedule to take shape.Project planning, as it relates to the scheduling process, has been defined by the PMBoK as:“The identification of the project objectives and the ordered activity necessary to complete the project including the identification of resource types and quantities required to carry out each activity or task.”
4Project SchedulingRepresents the conversion of project goals into an achievable methodology.Creates a timetable and reveals the network logic that relates project activities to each other.A graphical set of sequential relationships between project task which, when performed, result in the completion of the project goals.Vitally important to obtaining project goals, being on time and on budget.
5Project Network Diagrams (PND) Allows project teams to use a method for planning and schedulingThere are several advantages when project networks and scheduling are done wellShow interdependenceHelp schedule resourcesFacilitate communicationShow start & finish dates for taskIdentify critical activitiesDetermine project completionSee slide 26 for an example
7Most Common Methods for Constructing Activity Networks AOA vs. AONThe same mini-project is shown with activities on arrow…ECDBF…and activities on node.EDBFC
8Rules for Developing Activity Networks Some determination of activity precedence ordering must be done prior to creating the network.Network diagrams usually flow from left to right.An activity cannot begin until all preceding connected activities have been completed.Arrows on networks indicate precedence and logical flow. Arrows can cross over each other, although it is helpful for clarity’s sake to limit this effect when possible.Each activity should have a unique identifier associated with it (number, letter, code, etc.).Looping, or recycling through activities, is not permitted.Although not required, it is common to start a project on a single node. A single node point also is typically used as a project end indicator.
9Example of Creating a Project Activity Network Information for Network Construction Name: Project DeltaActivity Description PredecessorsA Contract signing NoneB Questionnaire design AC Target market ID AD Survey sample B, CE Develop presentation BF Analyze results DG Demographic analysis CH Presentation to client E, F, GConstruct a Network Diagram
10Activity Network Example Dev. Present.BDesignA ContractDSurveyF AnalysisH PresentCMarket IDGDemog.
11Project Activities Linked In Series/Parallel (Concurrent) College research paper example
13Node Labels Early Start Activity Float Activity Descriptor Late Start ID NumberActivity DurationLate FinishEarly FinishES ID EFSlack Task NameLS Duration LFDSurvey
14Duration Estimation Assumptions Methods Based on normal working methods during normal hoursDurations are always somewhat uncertainTimeframes can be from minutes to weeksMethodsPast experienceExpert opinionMathematical derivation based on Beta DistributionMost optimistic (a) time – better then plannedMost likely (m) time – realistic expectationMost pessimistic (b) time – Murphy’s Law kicks inThere are only two types of estimates…lucky and wrong.
15Variability in Activity Times Critical Path Method (CPM) assumes we know a fixed time estimate for each activity and there is no variability in activity timesProgram Evaluation and Review Technique (PERT) uses a probability distribution for activity times to allow for variability
16Activity Duration Estimation – Beta Distribution Where: a = Most optimistic time m = Most likely time b = Most pessimistic time
17Probability of Project Completion Project variance is computed by summing the variances of activities on the critical pathProject variance = (variances of activities on critical path)Project standard deviation = Project Variance
18Network Example Task Predecessor a m b mean variance Z -- 7 8 15 9.00 1.78Y13161916.001.00X14182218.00WY, X1214.000.44V145.004.00T61010.00ST, V1114.33Task a m b Mean VarianceZYXWVTSTask Early Start Early Finish Late Start Late Finish SlackZYXWVTSProject Length 64Project VarianceProject Std.devDetermine the expected duration and variance of each activity.Sketch the network described in the table.Determine the expected project time and standard deviation.
19Constructing the Network Paths Forward pass – an additive move through the network from start to finishBackward pass – a subtractive move through the network from finish to startCritical path – the longest path from end to end which determines the shortest project length
20Constructing the Critical Path Example Activity Description Predecessors Estimated Duration A Contract signing None 5 B Questionnaire design A 5 C Target market ID A 6 D Survey sample B, C 13 E Develop presentation B 6 F Analyze results D 4 G Demographic analysis C 9 H Presentation to client E, F, G 2Construct the critical path.
21Activity Network with Task Durations and Critical Path BDesign5EDev. Present6AContract5DSurvey13FAnalysis4HPresent2CMarket ID6GDemog.9Critical Path is indicated in bold
22Rules for Forward/Backward Pass Forward Pass Rules (ES & EF)ES + Duration = EFEF of predecessor = ES of successorLargest preceding EF at a merge point becomes ES for successorBackward Pass Rules (LS & LF)LF – Duration = LSLS of successor = LF of predecessorSmallest succeeding LS at a burst point becomes LF for predecessorES ID EFSlack Task NameLS Duration LFCalculate the forward/backwards pass.
23Activity Network with Forward Pass Added BDesign5EDev. Present6AContract5DSurvey13FAnalysis4HPresentation2CMarket ID6GDemograph.9ES ID EFSlack Task NameLS Duration LF
24Activity Network With Backward Pass Added DesignEDev. PresentAContractDSurveyFAnalysisHPresentationCMarket IDGDemograph.ES ID EFSlack Task NameLS Duration LF
25Activity Float - Slack Time Informs us of the amount an activity can be delayed without delaying the overall project.It is determined as a result of performing the forward and backward pass through the network.Calculated either byLF-EF = SlackLS-ES = SlackThe critical path is the network path with “0” slack.**This assumes a deadline has not been set for LF that is within our calculated project time.*Negative float is a result of the project time being longer than a set project end time.Calculate the slack time and determine critical path.
26Critical Path is indicated in bold Completed Activity Network With Critical Path And Activity Slack Times IdentifiedBDesignEDev. PresentAContractDSurveyFAnalysisH0 PresentationCritical Path is indicated in boldC0 Market IDG8 Demograph.ES ID EFSlack Task NameLS Duration LF
27Example Sketch the network described in the table. Determine the ES, LS, EF, LF, and slack of each activity.Determine the critical path.TaskPredecessorTimeA--4B9C11D5E3F7GD, FHE, G2K1Task ES EF LS LF SlackABCDEFGHK
28Laddering Activities ABC=18 days Laddered ABC=12 days A(3) B(6) C(9) Project ABC can be completed more efficiently if subtasks are used.Example: A does not need to be completely finished before work on B starts.ABC=18 daysA(3)B(6)C(9)A1(1)A2(1)A3(1)B1(2)B2(2)B3(2)C1(3)C2(3)C3(3)Laddered ABC=12 days
29Hammock Activities Used as a summary for subsets of activities 0 A 5 B 1515 C 180 Hammock 18Useful with a complex project or one that has a shared budget
30Reducing the Critical Path Eliminate tasks on the critical pathremove task with no valueConvert serial paths to parallel when possibleOverlap sequential tasksuse laddering when possibleShorten the duration on critical path tasksShortenearly tasks (have you read “The Goal”)longest taskseasiest taskstasks that cost the least to speed up – “crashing”
31Discussion QuestionsDefine the following terms: Path, Activity, Early start, Early finish, Late start, Late finish, Forward pass, Backward pass, Node, AON, Float, Critical Path, PERTDistinguish between serial activities and concurrent activities. Why do we seek to use concurrent activities as a way to shorten a project’s length?List three methods for deriving duration estimates for project activities. What are the strengths and weaknesses associated with each method?
32Discussion QuestionsIn your opinion, what are the chief benefits and drawbacks of using beta distribution calculations (based on PERT techniques) to derive activity duration estimates?“The shortest total length of a project is determined by the longest path through the network.” Explain the concept behind this statement. Why does the longest path determine the shortest project length?The float associated with each project task can only be derived following the completion of the forward and backward passes. Explain why this is true.
33Case Study 5Now that Joe has agreed to your WBS, he wants to review a schedule and present it to the president. She is a “big picture” thinker and does not usually get involved with the details, so Joe wants to limit the content of the diagram you show her to the basics that concern her.You have also worked with your team on estimating the durations of each of the work packages in the WBS. Note that in this case, the work packages are the scheduled activities. Here is your current plan for the briefing to the president:
35Case Study 5 Build a network diagram Calculate forward pass, backward pass, float, and critical pathBe ready to address—How long will the project take?When should you begin installing new furniture, communications equipment, and computers if you want to be in the new office by July 31?What items are on the critical path?