Critical Chain as an Extension to C P M


1
Critical Chain as an Extension to C P M

• Orlando A. Moreno PMP
• February 2003
• 408.656.2498

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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

3
Scheduling Issues Addressed by Critical Chain

• Inherent uncertainty in task duration estimates
• Parkinsons Law
• Work expands to fill the available time.
• Student Syndrome
• Wait until the last minute to start a task.

4
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

5
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
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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
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Putting Feeding Buffers in All Paths
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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
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Mean as a Reasonable Approximation to Median
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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
  

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Example Path Calculations
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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

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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

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Simulating Buffers in Microsoft Project
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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