vmcg 2/11/02 critical chain as an extension to cpm vincent mcgevna, pmp february 11, 2002...
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VMcG 2/11/02
Critical Chain as an Extension to CPM
Vincent McGevna, PMP
February 11, 2002
VMcG 2/11/02
Overview
• Introduction to Critical Chain
• The Use of Buffers for Contingency
• Sizing Tasks and Buffers
• Simulating Buffers in Microsoft Project
• Considerations When Using Critical Chain
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Scheduling Issues Addressed by Critical Chain
• Inherent uncertainty in task duration estimates• Parkinson’s Law:
Work expands to fill the available time.• Student Syndrome:
Wait until the last minute to start a task.
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Critical Chain Approach to Scheduling
• Account for both resource and precedence dependencies
• Set task duration for 50% probability of completing on time
• Add contingency with strategically placed buffers
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A Simple Example Will Illustrate the Use of Buffers
• Subsystem A requires six tasks (1 - 6)
• Subsystem B requires four tasks (7 - 10)
• Integration and test task (11)
• Subsystem A is on the critical path
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Network Diagram
1 2 3 4 5 6
8 9 107
11 End
A
B
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Critical Chain Adds Buffers
• Feeding Buffer Provides contingency to keep tasks not on the critical path off the critical path
• Project Buffer Provides contingency for the entire project
• Resource Buffer Provides a wakeup call to alert resources to be ready to work on critical tasks
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Network With Buffers Added
1 2 3 4 5 6
7 8 9 10Feeding
Buffer
11ProjectBuffer
ResourceBuffer
End
A
B
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Putting Feeding Buffers in All Paths
1 2 3 4 5 6
7 8 9 10Feeding
Buffer
11ProjectBuffer
ResourceBuffer
End
FeedingBuffer
A
B
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Task Sizing
• Create three estimates– Most Likely
– Optimistic
– Pessimistic
• Calculate mean• Use mean as an approximation for median
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Three Point Estimates with Mean
Optimistic PessimisticMost Likely
Mean
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Mean as a Reasonable Approximation to Median
Optimistic MostLikely
Pessimistic BetaDistribution
TriangleDistribution
Mean Median Mean Median5 10 15 10 10 10 105 10 20 11 11 12 115 10 25 12 11 13 135 10 35 13 13 17 16
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Buffer Sizing
• Contingency based on standard deviation of total path
• Setting buffer to one standard deviation increases probability of completing on-time to 84%
• 2.... taskpath devstddevstd
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Example Path Calculations
Activity Name Optimistic Most Likely Pessimistic Mean Std.Dev.
Variance
Initial draftGather information 8 9 16 11.0 1.8 3.2Write sections 7 10 20 12.3 2.8 7.7Review informally 2 3 6 3.7 .8 .7
Total: 22 27 3.4 11.6
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Simulating Buffers in Microsoft Project
• Add a task for the buffer at the end of the path• Add a milestone after the buffer
• Set buffer duration based on calculations
Task MilestoneBufferTask. . .
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Simulating Buffers in Microsoft Project
• Constrain the task type of the milestone to: Must Finish On
• Change the buffer to a milestone • Constrain the buffer task type to:
As Soon As Possible. • The slack for the buffer will now be the buffer size
Task MilestoneBufferTask. . .
VMcG 2/11/02
Simulating Buffers in Microsoft Project
VMcG 2/11/02
Considerations When Using Critical Chain
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Statistical analysis assumes many tasks with small random variations
Open ended tasks - the pessimistic estimate is significantly greater than the most likely - require special focus
– Identify and include in risk management plan– Consider changes to cost and scope to reduce
uncertainty– Consider an iterative development strategy – Estimate range of possible completion dates
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Statistical analysis assumes paths are not coupled
• Interim deliverables may be needed by other developers,
• Multiple strongly coupled paths could impact the statistical calculations
• Identify coupling and add to risk management plan
• Consider Monte Carlo simulation when there is a high degree of coupling
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Getting the most benefit from 3-point estimates
• Create 3-point estimates in early top level schedule
• Identify all assumptions about most likely, optimistic and pessimistic estimates
• Assess cost and scope impact of these assumptions
• Include most pessimistic completion estimate, cost and scope in all negotiations about schedule