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

VMcG 2/11/02 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.

VMcG 2/11/02 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

VMcG 2/11/02 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

VMcG 2/11/02 Network Diagram

VMcG 2/11/02 Critical Chain Adds Buffers Feeding BufferProvides contingency to keep tasks not on the critical path off the critical path Project BufferProvides contingency for the entire project Resource BufferProvides a wakeup call to alert resources to be ready to work on critical tasks

VMcG 2/11/02 Network With Buffers Added

VMcG 2/11/02 Putting Feeding Buffers in All Paths

VMcG 2/11/02 Task Sizing Create three estimates –Most Likely –Optimistic –Pessimistic Calculate mean Use mean as an approximation for median

VMcG 2/11/02 Three Point Estimates with Mean

VMcG 2/11/02 Mean as a Reasonable Approximation to Median

VMcG 2/11/02 Buffer Sizing Contingency based on standard deviation of total path Setting buffer to one standard deviation increases probability of completing on-time to 84%

VMcG 2/11/02 Example Path Calculations

VMcG 2/11/02 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

VMcG 2/11/02 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

VMcG 2/11/02 Simulating Buffers in Microsoft Project

VMcG 2/11/02 Considerations When Using Critical Chain

VMcG 2/11/02 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

VMcG 2/11/02 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

VMcG 2/11/02 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