© mcgraw-hill/irwin 2004 information systems project managementdavid olson 8-1
TRANSCRIPT
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© McGraw-Hill/Irwin 2004
Information Systems Project Management—David Olson8-1
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© McGraw-Hill/Irwin 2004
Information Systems Project Management—David Olson8-2
Chapter 8: Network Scheduling Methods
Critical path method (CPM)
Buffers
Leveling & Smoothing
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Information Systems Project Management—David Olson8-3
why networks?
• Gantt charts don’t explicitly show task relationships
• don’t show impact of delays or shifting resources well
• network models clearly show interdependencies
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Information Systems Project Management—David Olson8-4
Logic Diagrams
• network of relationships
elements & relationships (sequence)this is ACTIVITY-ON-NODE
can have ACTIVITY-ON-ARC
researchwhat’s
been done
researchwhat needs
doing pickfinaltopic
internetresearch
write print
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Information Systems Project Management—David Olson8-5
Network Diagrams
• activity duration
• milestone
• immediate predecessors
identified by arrows leading into• durations can include in parentheses• dummy activities need for AOA networks
activity(duration)
milestone
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Information Systems Project Management—David Olson8-6
networks
• 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
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Information Systems Project Management—David Olson8-7
Project Scheduling
MODEL COMPONENTS• activities from WBS• predecessors what this activity waits on• durations how long
– durations are PROBABILISTIC– CPM DETERMINISTIC– PERT considers uncertainty, but UNREALISTIC– simulation
• all assume unlimited resources
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Information Systems Project Management—David Olson8-8
Critical Path Method
• INPUTS: activities, durations, immediate predecessors
• ALGORITHMforward pass schedule 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
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Information Systems Project Management—David Olson8-9
CPM Example
FORWARD PASSactivity duration predecessorA requirements analysis 3 weeks -B programming 7 weeks AC get hardware 1 week AD train users 3 weeks B, Cschedule A start 0 finish 0+3 =3schedule B 3 3+7 =10
& C 3 3+1 =4schedule D 10 10+3 =13
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Information Systems Project Management—David Olson8-10
CPM Example
backward passschedule D finish 13 late start= 13-3 = 10schedule B 10 10-7 = 3
& C 10 10-1 = 9schedule A 3 3-3 = 0slack A LF= 3 EF= 3 3-3 = 0
B LF= 10 EF= 10 10-10= 0C LF= 10 EF= 4 10-4 = 6D LF= 13 EF= 13 13-13= 0
critical path: A-B-D
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Information Systems Project Management—David Olson8-11
Gantt Chart
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
A x x x
B x x x x x x x
C x
D x x x
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Information Systems Project Management—David Olson8-12
CPM
• can have more than one critical pathactivity duration predecessorA requirements analysis 3 weeks -B programming 7 weeks AC get hardware 7 weeks AD train users 3 weeks B, C
• critical paths A-B-DA-C-D
both with duration of 13 weeks
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Information Systems Project Management—David Olson8-13
Buffers
• 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
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Information Systems Project Management—David Olson8-14
Project Buffer
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
A x x x
B x x x x x x x
C x
D x x x b b
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Resource Limitations
critical path crashing
(cost/time tradeoff)
other methods
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Crashing
• 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
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Time Reduction
• 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
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Crash Example
Activity: programming
Tn: 7 weeks
Cn: $14,000 (7 weeks, 2 programmers)if you add a third programmer, done in 6 weeksTc: 6 weeks
Cn: $15,000
cost slope = (15000-14000)/(6-7)=-$1000/week
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Example Problem
activity Pred TnCn Tc Cc slope maxA requirements none 3 can’t crashB programming A 7 14000 6 15000 -1000 1 weekC get hardware A 1 50000 .5 51000 -2000 .5
weekD train users B,C 3 can’t crashCrashing Algorithm:1 crash only critical activities B only choice2 crash cheapest currently critical B is cheapest
3 after crashing one time period, recheck critical
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Crash Example
Import critical software from Australia: late penalty $500/d > 12 dA get import license 5 days no predecessorB ship 7 days A is predecessorC train users 11 days no predecessorD train on system 2 days B,C predecessorscan 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
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Crash Example
Original schedule: 14 days, $1,000 in penalties = $1000
crash B to 6 days:13 days, $500 penalties, $300 cost = $800*
crash B to 5C to 10: 12 days, no penalties, $400+2000 cost = $2400
to 11 days is worseNOW A SELECTION DECISIONrisk versus cost
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Crashing Limitations
• 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
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Resource Constraining
• 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
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Resource Leveling
unleveled leveled
1stQtr
2ndQtr
3rdQtr
4thQtr
0
5
10
15
20
25
1stQtr
2ndQtr
3rdQtr
4thQtr
analystsprogrammerstrainers
1stQtr
2ndQtr
3rdQtr
4thQtr
0
5
10
15
20
25
30
35
40
1stQtr
2ndQtr
3rdQtr
4thQtr
analysts
programmers
trainers
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Work Patterns
• 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
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Resource Leveling
this leveling often works for specific activities, but complicated even more when resources shared
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Resource Leveling Methods
• split up work, stagger
• eliminate some activities (subcontract)
• substitute less resource consuming activities (use CASE tools)
• substitute resources (hire spot work programmers)
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Resource Smoothing
• 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
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Resource Loading
• 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
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Recap
• 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
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Criticisms of CPM
• 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
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Summary
• 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