# Backward Scheduling for Projects Jared Cummings Marriott School Brigham Young University.

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Backward Scheduling for Projects Jared Cummings Marriott School Brigham Young University

Preview Slide Definition of backward scheduling3 Ways to use it in your business4 The basics6 How backward scheduling works7 Real world example9 Exercise15 Summary19 Readings list20

Definition Backward scheduling refers to the process of planning a project by commencing with the deadline and designating the component steps in reverse order of time, ultimately revealing the latest possible start date.

How can you use backward scheduling in your business? First ask yourself what benefits you can gain by starting a project as late as possible. Are there any incentives to finishing the new project early? Are needed resources currently occupied by other projects?

Not just for manufacturing… Audits Performance appraisals Consumer testing for marketing Research & Development Or any other process that has multiple steps and a due date

The Basics Gantt chart Activity-on-node (AON) or Activity-on-arrow (AOA) network diagram PERT Specialized software

AON diagram A 08 1 9 8 Activity name Early Finish Time Late Finish Time Early Start Time Late Start Time Activity duration NodeDiagram

PERT Program Evaluation and Review Technique is used to obtain realistic estimates of the duration of each component step. Where o = optimistic duration, m = most likely duration, and p = pessimistic duration, the formula is: (o + 4m + p) 6

Real world example Joe Zoobie needs to prepare a presentation for his Operations Management class. The due date is five days from now at 8 am. Each day Joe has four hours, from noon to 4 pm, to do homework. Joe knows that there are several steps to take in order to prepare the presentation, but he has homework in all his other classes too, and some other assignments are due before the OM presentation. He needs to find the latest hour that he can start on the presentation so that he can devote enough time to his other assignments first.

Here are the precedence relationships and the o, m, and p durations (in hours) for each component step of Joe’s project: 0.17 0.08 IBurn presentation on CDJ 1.751.000.50F, G, HCompile and order slidesI 6.003.502.00ECreate slide contentH 0.750.250.10EWork example calculationsG 2.000.500.25EFind stock photos for slidesF 1.500.500.25DCreate outlineE 10.005.003.00B, CRead articles and textsD 2.001.000.75AFind source texts in libraryC 4.002.001.00AFind source articles onlineB 0.500.080.02noneChoose topicA pmoPredecessorsActivity 0.10 1.04 3.67 0.31 0.71 0.63 5.50 1.13 2.17 0.14 d

The next step is to draw the AON diagram to show the durations and predecessors of each activity: A C D B E H G F I J 0.14 2.17 1.13 5.500.63 0.71 0.31 3.67 1.040.10

Now, start at the first activity (or activities, if you have concurrent starting activities) and add the d value to each activity’s EST in order to get each activity’s EFT. The EST for a starting activity is zero. The EST for all other activities is equal to the EFT of their immediate predecessors. Note that if an activity has convergent predecessors then you must choose the latest of the predecessors’ EFT’s to be the EST of the subsequent activity. If one predecessor diverges into several activities, then the EST of all the subsequent activities will equal the EFT of the predecessor. A C D B E H G F I J 0.14 2.17 1.13 5.500.63 0.71 0.31 3.67 1.040.10 7.810.14 1.27 2.31 8.43 9.14 8.74 12.10 13.1413.240.0 0.14 2.31 0.14 7.81 8.43 12.1013.14 8.43

In the previous step we found the total time needed to complete Joe’s project: 13.24 hours, which happens to be the EFT of the last activity. If we subtract 13.24 from the 20 total hours that Joe has to do his homework, then we find that Joe must start his OM presentation 6.76 hours from now, i.e. at 2:46 pm tomorrow. Joe also wants to find the LST for each activity, so that he can see if he’s staying on schedule. So we must reverse the previous step… A C D B E H G F I J 0.14 2.17 1.13 5.500.63 0.71 0.31 3.67 1.040.10 7.810.14 1.27 2.31 8.43 9.14 8.74 12.10 13.1413.240.0 0.14 2.31 0.14 7.81 8.43 12.1013.14 8.43

Start at the last activity (or activities, if you have concurrent ending activities) and subtract the d value from each activity’s LFT in order to get each activity’s LST. The LFT for activity J is 20. The LFT for all other activities is equal to the LST of their immediate successors. Note that if an activity has a divergent predecessor then you must choose the earliest of the successors’ LST’s to be the LFT of the previous activity. If several predecessors converge into one activity, then the LFT of all the predecessors will equal the LST of the subsequent activity. A C D B E H G F I J 0.14 2.17 1.13 5.500.63 0.71 0.31 3.67 1.040.10 7.810.14 1.27 2.31 8.43 9.14 8.74 12.10 13.1413.240.0 0.14 2.31 0.14 7.81 8.43 12.1013.14 8.43 6.91 9.07 14.5715.20 18.86 19.9120.06.76 7.95 9.07 6.91 14.57 15.20 18.56 18.16 18.8619.91

Exercise Juana Fulana is the forewoman for a construction project. She is planning a parking garage for the local university. The project must be completed by September 1, 2005, when the fall semester starts. Today is July 1, 2005, so she has only 43 work days in which to finish the project. Since her construction team could be working on other projects as well, Juana would like to start the parking garage on the last possible day. What day is that?

Here are the project activities, with precedence relationships and o, m, and p times. Use them to find d, LFT, and LST for each activity. Activity Predecessors omp ASurvey sitenone123 BExcavate the building areaA4510 CLay foundations and build frameB1912 DInstall doorsC112 EPaintingC123 FElectricalC236 GCommunication and peripheral systemsF031 HFinalize buildingD, E, G123 IExterior landscapingH457 JCleanupI124 The answers are on the next two slides— don’t look until you’ve done the exercise yourself.

A C D BE H G F I J 5.67 2.00 8.13 1.17 2.00 3.332.14 2.005.17 Answer: AON diagram 2.17 33.67 16.96 33.67 21.27 35.67 23.27 35.67 23.27 40.83 28.43 40.83 28.43 43 30.60 33.67 17.79 28.19 15.79 31.53 19.13 31.53 19.13 33.67 21.27 20.07 7.67 28.19 15.79 20.07 7.67 31.67 15.79 32.50 15.79 12.40 0 14.40 2.00 14.40 2.00 Juana must have her team start the project on the twelfth day (July 19, 2005).

Summary Backward scheduling is a fairly simple process that can be carried out with no special software. In spite of its simplicity, backward scheduling is one of the most essential tools of project management. Remember that backward scheduling can be used for almost any project in order to create an accurate schedule and keep to it.

Readings list Some good places to look for more material about backward scheduling (and scheduling in general) are construction management books and the tutorial books meant to accompany project management software, as well as online sources. Here are some sources that I used: Marchman, David A. and Terry L. Anderson. Construction Scheduling with Primavera Enterprise. Clifton Park, NY: Thompson Delmar Learning, 2003. Naylor, Henry. Construction Project Management: Planning and Scheduling. Albany, NY: Delmar Publishers, 1995. “Program Evaluation and Review Technique." Wikipedia. 26 Nov. 2005.

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