Title: Critical Chain as an Extension to C P M
1Critical Chain as an Extension to C P M
- Orlando A. Moreno PMP
- February 2003
- 408.656.2498
2Overview
- Introduction to Critical Chain
- The Use of Buffers for Contingency
- Sizing Tasks and Buffers
- Simulating Buffers in Microsoft Project
- Considerations When Using Critical Chain
3Scheduling 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.
4Critical 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
5A 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
6Network Diagram
7Critical 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
8Network With Buffers Added
9Putting Feeding Buffers in All Paths
10Task Sizing
- Create three estimates
- Most Likely
- Optimistic
- Pessimistic
- Calculate mean
- Use mean as an approximation for median
11Three Point Estimates with Mean
12Mean as a Reasonable Approximation to Median
13Buffer Sizing
- Contingency based on standard deviation of total
path
- Setting buffer to one standard deviation
increases probability of completing on-time to
84
-
14Example Path Calculations
15Simulating 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
16Simulating 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
17Simulating Buffers in Microsoft Project
18Considerations When Using Critical Chain
19Statistical 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
20Statistical 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
21Getting 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