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Chapter 8: Network Scheduling Methods

Chapter 8: Network Scheduling Methods. Critical path method ( CPM ) Buffers Leveling & Smoothing. Gantt charts don’t explicitly show task relationships don’t show impact of delays or shifting resources well network models clearly show interdependencies. why networks?.

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Chapter 8: Network Scheduling Methods

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  1. Chapter 8: Network Scheduling Methods Critical path method (CPM) Buffers Leveling & Smoothing

  2. Gantt charts don’t explicitly show task relationships don’t show impact of delays or shifting resources well network models clearly show interdependencies why networks?

  3. network of relationships elements & relationships (sequence) this is ACTIVITY-ON-NODE can have ACTIVITY-ON-ARC Logic Diagrams research what’s been done research what needs doing pick final topic write print internet research

  4. activity duration milestone immediate predecessors identified by arrows leading into durations can include in parentheses dummy activities need for AOA networks Network Diagrams activity (duration) milestone

  5. networks make a good visual they are TOTALLY UNNECESSARY for identifying early starts earliest an activity can be begun late finishes latest an activity can finish slack spare time critical paths activities with no slack networks

  6. MODEL COMPONENTS activitiesfrom WBS predecessorswhat this activity waits on durations how long durations are PROBABILISTIC CPM DETERMINISTIC PERT considers uncertainty, but UNREALISTIC simulation all assume unlimited resources Project Scheduling

  7. INPUTS: activities, durations, immediate predecessors ALGORITHM forward passschedule all activities with no unscheduled predecessors ES/EF determine early starts & early finishes (start ASAP, add duration) backwards pass schedule in reverse (schedule all activities with no unscheduled FOLLOWERS) LF/LS determine late finishes, subtract duration to get late starts slack difference between LS-ES (same as LF-EF) critical path all chains of activities with no slack Critical Path Method

  8. FORWARD PASS activity duration predecessor A requirements analysis 3 weeks - B programming 7 weeks A C get hardware 1 week A D train users 3 weeks B, C schedule A start 0 finish 0+3 =3 schedule B 3 3+7 =10 & C 3 3+1 =4 schedule D 10 10+3 =13 CPM Example

  9. backward pass schedule D finish 13 late start= 13-3 = 10 schedule B 10 10-7 = 3 & C 10 10-1 = 9 schedule A 3 3-3 = 0 slack A LF= 3 EF= 3 3-3 = 0 B LF= 10 EF= 10 10-10 = 0 C LF= 10 EF= 4 10-4 = 6 D LF= 13 EF= 13 13-13 = 0 critical path: A-B-D CPM Example

  10. Gantt Chart

  11. can have more than one critical path activity duration predecessor A requirements analysis 3 weeks - B programming 7 weeks A C get hardware 7weeks A D train users 3 weeks B, C critical paths A-B-D A-C-D both with duration of 13 weeks CPM

  12. Assure activities completed on time (Goldratt, 1997) Project Buffers:after final project task Feeding Buffers: where non-critical activities lead into critical activities Resource Buffers: before resources scheduled to work on critical activities Strategic Resource Buffers: assure key resources available Buffers

  13. Project Buffer

  14. Resource Limitations critical path crashing (cost/time tradeoff) other methods

  15. can shorten project completion time by adding extra resources (costs) start off with NORMAL TIME CPM schedule get expected duration Tn, cost Cn Tn should be longest duration Cn should be most expensive in penalties, cheapest in crash costs Crashing

  16. to reduce activity time, pay for more resources develop table of activities with times and costs for each activity, usually assume linear relationship for relationship between cost & time Time Reduction

  17. Activity: programming Tn: 7 weeks Cn: $14,000(7 weeks, 2 programmers) if you add a third programmer, done in 6 weeks Tc: 6 weeks Cn: $15,000 cost slope = (15000-14000)/(6-7)=-$1000/week Crash Example

  18. activity Pred Tn Cn Tc Cc slope max A requirements none 3 can’t crash B programming A 7 14000 6 15000 -1000 1 week C get hardware A 1 50000 .5 51000 -2000 .5 week D train users B,C 3 can’t crash Crashing Algorithm: 1 crash only critical activitiesB only choice 2 crash cheapest currently criticalB is cheapest 3 after crashing one time period, recheck critical Example Problem

  19. Import critical software from Australia: late penalty $500/d > 12 d A get import license 5 days no predecessor B ship 7 days A is predecessor C train users 11 days no predecessor D train on system 2 days B,C predecessors can crash C: $2000/day more than current for up to 3 days B: faster boat 6 days $300 more than current bush plane 5 days $400 more than current commercial 3 days $500 more than current Crash Example

  20. Original schedule: 14 days, $1,000 in penalties = $1000 crash B to 6 days:13 days, $500 penalties, $300 cost = $800* crash B to 5 C to 10: 12 days, no penalties, $400+2000 cost = $2400 to 11 days is worse NOW A SELECTION DECISION risk versus cost Crash Example

  21. assumes linear relationship between time and cost not usually true (indirect costs don’t change at same rate as direct costs) requires a lot of extra cost estimation time consuming ends with tradeoff decision Crashing Limitations

  22. CPM & PERT both assume unlimited resources NOT TRUE may have only a finite number of systems analysts, programmers RESOURCE LEVELING - balance the resource load RESOURCE CONSTRAINING - don’t exceed available resources Resource Constraining

  23. unleveled leveled Resource Leveling

  24. natural resource demands tend to have lumps maintaining a stable work force works better if demand leveled HOW TO LEVEL: split each activity into smaller activities, schedule them at different times USUALLY NOT THAT EASY Work Patterns

  25. this leveling often works for specific activities, but complicated even more when resources shared Resource Leveling

  26. split up work, stagger eliminate some activities (subcontract) substitute less resource consuming activities (use CASE tools) substitute resources (hire spot work programmers) Resource Leveling Methods

  27. Adjust schedules to level workload expand duration for peak load compress durations where load low Fill in gaps of work Requires balancing resources for activities with heavier load, use multiple crews Resource Smoothing

  28. MUST schedule activities to not overschedule critical resource If there is only one training room, and it includes the only delivery system can’t speed up training can’t conduct two training activities at once LINEAR PROGRAMMING heuristics Resource Loading

  29. cost/time tradeoff time consuming, still makes assumptions resource leveling manual shuffling resource constraining pure solution to optimality a research issue heuristics have been applied in software NO IDEAL SOLUTION METHOD Recap

  30. Rarely to activities proceed as planned critical path therefore very volatile options to speed some activities available crashing resource limits not reflected resource leveling schedule likely to be very lumpy resource smoothing Criticisms of CPM

  31. Critical path provides managers valuable information What activities interfere with project completion Estimate of project duration Buffers a means to manage risk Crashing a means to analyze cost/time tradeoff Resource management Leveling smoothing Summary

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