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© 2008 Prentice-Hall, Inc. Chapter 13 To accompany Quantitative Analysis for Management, Tenth Edition, by Render, Stair, and Hanna Power Point slides created by Jeff Heyl Project Management © 2009 Prentice-Hall, Inc.
© 2009 Prentice-Hall, Inc. 13 – 2 Learning Objectives 1.Understand how to plan, monitor, and control projects with the use of PERT and CPM 2.Determine earliest start, earliest finish, latest start, latest finish, and slack times for each activity, along with the total project completion time 3.Reduce total project time at the least total cost by crashing the network using manual or linear programming techniques 4.Understand the important role of software in project management After completing this chapter, students will be able to:
© 2009 Prentice-Hall, Inc. 13 – 3 Chapter Outline Introduction PERT/CPM PERT/Cost Project Crashing Other Topics in Project Management
© 2009 Prentice-Hall, Inc. 13 – 4 Introduction Most realistic projects are large and complex Tens of thousands of steps and millions of dollars may be involved Managing large-scale, complicated projects effectively is a difficult problem and the stakes are high work breakdown structure The first step in planning and scheduling a project is to develop the work breakdown structure Time, cost, resource requirements, predecessors, and people required are identified for each activity Then a schedule for the project can be developed
© 2009 Prentice-Hall, Inc. 13 – 5 Introduction program evaluation and review technique PERTcritical path methodCPM The program evaluation and review technique (PERT) and the critical path method (CPM) are two popular quantitative analysis techniques to help plan, schedule, monitor, and control projects Originally the approaches differed in how they estimated activity times PERT used three time estimates to develop a probabilistic estimate of completion time CPM was a more deterministic technique They have become so similar they are commonly considered one technique, PERT/CPM
© 2009 Prentice-Hall, Inc. 13 – 6 1.Define the project and all of its significant activities or tasks 2.Develop the relationships among the activities and decide which activities must precede others 3.Draw the network connecting all of the activities 4.Assign time and/or cost estimates to each activity critical path 5.Compute the longest time path through the network; this is called the critical path 6.Use the network to help plan, schedule, monitor, and control the project Six Steps of PERT/CPM The critical path is important since any delay in these activities can delay the completion of the project
© 2009 Prentice-Hall, Inc. 13 – 7 PERT/CPM Given the large number of tasks in a project, it is easy to see why the following questions are important 1.When will the entire project be completed? critical 2.What are the critical activities or tasks in the project, that is, the ones that will delay the entire project if they are late? non-critical 3.Which are the non-critical activities, that is, the ones that can run late without delaying the entire project’s completion? 4.If there are three time estimates, what is the probability that the project will be completed by a specific date?
© 2009 Prentice-Hall, Inc. 13 – 8 PERT/CPM 5.At any particular date, is the project on schedule, behind schedule, or ahead of schedule? 6.On any given date, is the money spent equal to, less than, or greater than the budgeted amount? 7.Are there enough resources available to finish the project on time?
© 2009 Prentice-Hall, Inc. 13 – 9 General Foundry Example of PERT/CPM General Foundry, Inc. has long been trying to avoid the expense of installing air pollution control equipment The local environmental protection group has recently given the foundry 16 weeks to install a complex air filter system on its main smokestack General Foundry was warned that it will be forced to close unless the device is installed in the allotted period They want to make sure that installation of the filtering system progresses smoothly and on time
© 2009 Prentice-Hall, Inc. 13 – 10 General Foundry Example of PERT/CPM Activities and immediate predecessors for General Foundry ACTIVITYDESCRIPTION IMMEDIATE PREDECESSORS ABuild internal components— BModify roof and floor— CConstruct collection stackA DPour concrete and install frameB EBuild high-temperature burnerC FInstall control systemC GInstall air pollution deviceD, E HInspect and testF, G Table 13.1
© 2009 Prentice-Hall, Inc. 13 – 11 Drawing the PERT/CPM Network There are two common techniques for drawing PERT networks Activity-on-nodeAON Activity-on-node (AON) where the nodes represent activities Activity-on-arcAOA Activity-on-arc (AOA) where the arcs are used to represent the activities The AON approach is easier and more commonly found in software packages One node represents the start of the project, one node for the end of the project, and nodes for each of the activities The arcs are used to show the predecessors for each activity
© 2009 Prentice-Hall, Inc. 13 – 12 General Foundry Example of PERT/CPM Network for General Foundry A Build Internal Components H Inspect and Test E Build Burner C Construct Collection Stack Start F Install Control System Finish G Install Pollution Device D Pour Concrete and Install Frame B Modify Roof and Floor Figure 13.1
© 2009 Prentice-Hall, Inc. 13 – 13 Activity Times In some situations, activity times are known with certainty CPM assigns just one time estimate to each activity and this is used to find the critical path In many projects there is uncertainty about activity times PERT employs a probability distribution based on three time estimates for each activity A weighted average of these estimates is used for the time estimate and this is used to determine the critical path
© 2009 Prentice-Hall, Inc. 13 – 14 Activity Times The time estimates in PERT are Optimistic time a Optimistic time ( a ) = time an activity will take if everything goes as well as possible. There should be only a small probability (say, 1 / 100 ) of this occurring. Pessimistic time b Pessimistic time ( b ) = time an activity would take assuming very unfavorable conditions. There should also be only a small probability that the activity will really take this long. Most likely time m Most likely time ( m ) = most realistic time estimate to complete the activity
© 2009 Prentice-Hall, Inc. 13 – 15 Activity Times beta probability distribution PERT often assumes time estimates follow a beta probability distribution Probability of 1 in 100 of a Occurring Probability of 1 in 100 of b Occurring Probability Activity Time Most Likely Time (m)(m) Most Optimistic Time (a)(a) Most Pessimistic Time (b)(b) Figure 13.2
© 2009 Prentice-Hall, Inc. 13 – 16 Activity Times expected activity time t To find the expected activity time ( t ), the beta distribution weights the estimates as follows variance of activity completion time To compute the dispersion or variance of activity completion time, we use the formula
© 2009 Prentice-Hall, Inc. 13 – 17 Activity Times Time estimates (weeks) for General Foundry ACTIVITY OPTIMISTIC, a MOST PROBABLE, m PESSIMISTIC, b EXPECTED TIME, t = [( a + 4 m + b )/6] VARIANCE, [( b – a )/6] 2 A12324/36 B2343 C1232 D246416/36 E147436/36 F129364/36 G /36 H12324/36 25 Table 13.2
© 2009 Prentice-Hall, Inc. 13 – 18 How to Find the Critical Path We accept the expected completion time for each task as the actual time for now The total of 25 weeks in Table 13.2 does not take into account the obvious fact that some of the tasks could be taking place at the same time To find out how long the project will take we perform the critical path analysis for the network critical path The critical path is the longest path through the network
© 2009 Prentice-Hall, Inc. 13 – 19 How to Find the Critical Path General Foundry’s network with expected activity times A2A2C2C2 H2H2E4E4 B3B3D4D4G5G5 F3F3 StartFinish Figure 13.3
© 2009 Prentice-Hall, Inc. 13 – 20 How to Find the Critical Path To find the critical path, need to determine the following quantities for each activity in the network 1.Earliest start timeES 1.Earliest start time (ES): the earliest time an activity can begin without violation of immediate predecessor requirements 2.Earliest finish timeEF 2.Earliest finish time (EF): the earliest time at which an activity can end 3.Latest start timeLS 3.Latest start time (LS): the latest time an activity can begin without delaying the entire project 4.Latest finish timeLF 4.Latest finish time (LF): the latest time an activity can end without delaying the entire project
© 2009 Prentice-Hall, Inc. 13 – 21 How to Find the Critical Path In the nodes, the activity time and the early and late start and finish times are represented in the following manner ACTIVITY t ESEF LSLF Earliest times are computed as Earliest finish time =Earliest start time + Expected activity time EF =ES + t Earliest start =Largest of the earliest finish times of immediate predecessors ES =Largest EF of immediate predecessors
© 2009 Prentice-Hall, Inc. 13 – 22 How to Find the Critical Path At the start of the project we set the time to zero Thus ES = 0 for both A and B Start A t = 2 ES = 0EF= = 2 B t = 3 ES = 0EF= = 3
© 2009 Prentice-Hall, Inc. 13 – 23 How to Find the Critical Path General Foundry’s ES and EF times A202A202 C224C224 H E448E448 B303B303 D437D437 G5 813 F347F347 StartFinish Figure 13.4
© 2009 Prentice-Hall, Inc. 13 – 24 How to Find the Critical Path Latest times are computed as Latest start time =Latest finish time – Expected activity time LS =LF – t Latest finish time =Smallest of latest start times for following activities LF =Smallest LS of following activities For activity H LS = LF – t = 15 – 2 = 13 weeks
© 2009 Prentice-Hall, Inc. 13 – 25 How to Find the Critical Path General Foundry’s LS and LF times A20202A20202 C22424C22424 H E44848E44848 B30314B30314 D43748D43748 G5 813 F StartFinish Figure 13.5
© 2009 Prentice-Hall, Inc. 13 – 26 How to Find the Critical Path slack time Once ES, LS, EF, and LF have been determined, it is a simple matter to find the amount of slack time that each activity has Slack = LS – ES, or Slack = LF – EF From Table 13.3 we see activities A, C, E, G, and H have no slack time critical activities critical path These are called critical activities and they are said to be on the critical path The total project completion time is 15 weeks Industrial managers call this a boundary timetable
© 2009 Prentice-Hall, Inc. 13 – 27 How to Find the Critical Path General Foundry’s schedule and slack times ACTIVITY EARLIEST START, ES EARLIEST FINISH, EF LATEST START, LS LATEST FINISH, LF SLACK, LS – ES ON CRITICAL PATH? A02020Yes B03141No C24240Yes D37481No E48480Yes F No G8138 0Yes H Yes Table 13.3
© 2009 Prentice-Hall, Inc. 13 – 28 How to Find the Critical Path General Foundry’s critical path A20202A20202 C22424C22424 H E44848E44848 B30314B30314 D43748D43748 G5 813 F StartFinish Figure 13.6
© 2009 Prentice-Hall, Inc. 13 – 29 Probability of Project Completion critical path analysis The critical path analysis helped determine the expected project completion time of 15 weeks But variation in activities on the critical path can affect overall project completion, and this is a major concern If the project is not complete in 16 weeks, the foundry will have to close PERT uses the variance of critical path activities to help determine the variance of the overall project Project variance = ∑ variances of activities on the critical path
© 2009 Prentice-Hall, Inc. 13 – 30 Probability of Project Completion From Table 13.2 we know that ACTIVITYVARIANCE A4/36 B C D16/36 E36/36 F64/36 G H4/36 Hence, the project variance is Project variance = 4 / / / / / 36 = 112 / 36 = 3.111
© 2009 Prentice-Hall, Inc. 13 – 31 Probability of Project Completion We know the standard deviation is just the square root of the variance, so We assume activity times are independent and total project completion time is normally distributed
© 2009 Prentice-Hall, Inc. 13 – 32 Probability of Project Completion Probability distribution for project completion times Standard Deviation = 1.76 Weeks (Expected Completion Time) 15 Weeks Figure 13.7
© 2009 Prentice-Hall, Inc. 13 – 33 Probability of Project Completion The standard normal equation can be applied as follows From Appendix A we find the probability of associated with this Z value That means there is a 71.6% probability this project can be completed in 16 weeks or less
© 2009 Prentice-Hall, Inc. 13 – 34 Probability of Project Completion Probability of General Foundry meeting the 16- week deadline 0.57 Standard Deviations Time 15 Weeks Figure Weeks Expected Time is 15 Weeks Probability ( T ≤ 16 Weeks) is 71.6%
© 2009 Prentice-Hall, Inc. 13 – 35 What PERT Was Able to Provide PERT has been able to provide the project manager with several valuable pieces of information The project’s expected completion date is 15 weeks There is a 71.6% chance that the equipment will be in place within the 16-week deadline Five activities (A, C, E, G, H) are on the critical path Three activities (B, D, F) are not critical but have some slack time built in A detailed schedule of activity starting and ending dates has been made available
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