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Introduction to Critical Chain

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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Critical Chain Presentation Critical Chain Presentation Presentation Transcript

  • Critical Chain as an Extension to CPM Orlando A. Moreno, PMP February 11, 2003 [email_address]
  • 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
    Orlando Moreno 2/11/03 408.656.2498
  • 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.
    Orlando Moreno 2/11/03 408.656.2498 View slide
  • 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
    Orlando Moreno 2/11/03 408.656.2498 View slide
  • 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
    Orlando Moreno 2/11/03 408.656.2498
  • Network Diagram Orlando Moreno 2/11/03 408.656.2498
  • 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
    Orlando Moreno 2/11/03 408.656.2498
  • Network With Buffers Added Orlando Moreno 2/11/03 408.656.2498
  • Putting Feeding Buffers in All Paths Orlando Moreno 2/11/03 408.656.2498
  • Task Sizing
    • Create three estimates
      • Most Likely
      • Optimistic
      • Pessimistic
    • Calculate mean
    • Use mean as an approximation for median
    Orlando Moreno 2/11/03 408.656.2498
  • Three Point Estimates with Mean Orlando Moreno 2/11/03 408.656.2498
  • Mean as a Reasonable Approximation to Median Orlando Moreno 2/11/03 408.656.2498
  • Buffer Sizing
    • Contingency based on standard deviation of total path
    • Setting buffer to one standard deviation increases probability of completing on-time to 84%
    Orlando Moreno 2/11/03 408.656.2498
  • Example Path Calculations Orlando Moreno 2/11/03 408.656.2498
  • 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
    Orlando Moreno 2/11/03 408.656.2498
  • 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
    Orlando Moreno 2/11/03 408.656.2498
  • Simulating Buffers in Microsoft Project Orlando Moreno 2/11/03 408.656.2498
  • Considerations When Using Critical Chain Orlando Moreno 2/11/03 408.656.2498
  • 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
    Orlando Moreno 2/11/03 408.656.2498
  • 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
    Orlando Moreno 2/11/03 408.656.2498
  • 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
    Orlando Moreno 2/11/03 408.656.2498
  • Questions Orlando Moreno [email_address] 408.656.2498 408.656.2498 [email_address]