Project Estimation Strategies

The challenge of estimation

• project level estimation not task level estimation
  
• theory of constrains

Critical Path Method and Its Limitation

• critical chain = CPM + resource contrains + uncertainty

How to identify critical chain

• identify critical chain after resource leveling
  
• critical chain
• second chain

Popular Estimation Strattegies

• current estimate = expected right answer + permissible slip
• current estimate = last estimate
• current estimate = last estimate + permissible slip
• current estimate = last estimate - a few days shaving

The reasons for poor estimation

• accurate task estimation is difficult.
• current project doesn't resemble previous projects
• detailed records are often not maintained

Project padding and task padding

Using buffers successfully

• add buffers to absorb variance
• estimate task times as likely duration
• evaluate the project status by reviewing g buffers, not individual task.
• buffers != Delayed Finish Date

Estimate task times as likely durations
Evaluate the project status by reviewing buffers, not individual tasks
Three buffer types

• project buffer: a block of time placed at the end of a project's Critical Chain
  
• feeding buffer: a block of time placed at the end of a secondary chain of tasks at the point where it feeds into the Critical Chain
  
• resource buffer: A strtegy of assuring a resource's availability for certain key tasks

Estimation for Innovative Project

• Base duration estimates on likely durations
• Add sufficient buffers to task chains

- large enough to absorb errors
- small enough to avoid unneccessary delays
Some Rules-of-thumb for Buffer Sizes

• Project Buffer: 50% of the unpadded Critical Chian
• Feeding Buffer: 50% of the saved padding
• Resource Buffer: 2 weeks notification

Sizing Buffers based on Task Types

• 4 types of tasks Repetition 10% Variation 25% Innovation 50% Novelty

Unused buffer Time

• unused task padding is lost to the project
• unused buffer time permits accelerated schedules
• an unused project buffer means your project has completed ahead of schedule

Parkinson's law
Work expands to fill available time

As a rule-of-thumb, how far in advance should team members be notified before they are to begin a critical task? 2 weeks.

Building the critical chain schedule

• define the project

a clear project definition includes
- project objectives
- timing, scope and budget
- priorities

• develop a high-level WBS

• add detail level tasks

• enter task durations

• use likely durations when estimating

• calculate padding

• identify task dependencies

• calculate a tentative finish date

• add resources & resource leveling

• identify the critical chain

• shorten the critical chain

• identify the secondary chains

• calculate buffer sizes

• insert the project buffer and make adjustments

• Insert Feeding buffers and make adjustments

• monitor all buffers