How to create `Critical Chain' using MS Project & How to introduce buffers into it?

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Request for Question Clarification by joseleon-ga on 13 Aug 2002 11:00 PDT

Hello:
 Do you mean 'Critical Chain' or 'Critical Path'?

Regards.

Hello Vishwakarma,

Thanks for your question - I haven't heard anyone mention "Critical
Chain" since I read some of Goldratt's books on the theory of
constraints about five years ago. I will assume you understand the
basic concept of the critical chain and buffers and will point you to
some on line references that appear to solve the "how to schedule it"
problem.

A Critical Chain Schedule (A Better Wrong Answer)

http://www.focusedperformance.com/articles/ccsamp01.html

This page walks through a fourteen task schedule and describes the
methods used to build a schedule with buffers. Basically it...
 - does the critical path using minimum task durations
 - uses half of the "safety" (difference between realistic and
aggressive task durations) as feeding buffers
 - uses remaining safety as the "project buffer" at the end.
The end result is a 22 day duration schedule vs. the typical 24 day
duration for the critical path. There are a number of other good pages
on this site - I suggest browsing it for more suggestions.

To enter the plan as described in ccsamp01 into MS project, I would...
 - Enter the tasks as described above (minimum task durations, put in
the dependent links); you get a minimum duration task and can evaluate
which tasks you will manage as the critical chain
 - Introduce each feeding buffers by setting the start date for the
task, starting at the earliest task and finishing with the latest
task. The slack between tasks represent the feeding buffer duration.
 - Add a milestone w/ a fixed date to represent the planned end date
based on the project buffer.
That gets you a schedule that reflects the critical chain and buffers.
The buffer durations can be read off as slack.

Note that this description doesn't address the resource constraints
and use of resource buffers. I would introduce the resource
availability and buffers as milestones (similar to the project end) to
highlight the need of those resources and give you slack calcuations
you need for buffer management.

Of course, once you have the plan, you need to execute the plan and
track accomplishments to the plan. Use your actual start dates only
for the initial planning. The slack calcuation of a task under way can
be compared with the original buffer to determine if the task is
"green", "yellow", or "red" (its health). You also need to make sure
that the resources to do the tasks get the proper advance warning so
they can work on your tasks when the prerequisites are met.

For further searches, phrases such as "critical chain" and schedule.
One of the first hits I found was for a training class on this. I
didn't review the material so I can't comment on it further, but the
material was available without cost if you are interested.

Let me know if you need anything further clarified on this.
  --Maniac

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Request for Answer Clarification by vishwakarma-ga on 18 Aug 2002 23:01 PDT

Hello Maniac,

Thank you very much for answering my question.

I regret to say that,it could not fulfill my requirement.

The question was focussed on integration of Critical chain & MS
project

operation; the answer is limited to TOC aspect only.

The same is applicable to later part of the question i.e. introduction
of Buffer.

I expect how to create critical chain USING MS PROJECT & how to
introduce buffers into it.

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Clarification of Answer by maniac-ga on 19 Aug 2002 16:20 PDT

Hello Vishwakarma, 

You are requesting a clarification of how to introduce the buffers
into MS project. For this clarification, I will use the example at

A Critical Chain Schedule (A Better Wrong Answer) 
http://www.focusedperformance.com/articles/ccsamp01.html 

and start with the second illustration (showing the minimum task
durations, and tasks linked with no buffers). I will assume you have
entered this schedule into Microsoft Project with
 - the minimum task durations as described
 - the constraints between tasks
 - the start dates set for tasks F (day 1), D (day 1), A (day 3), C
and H (day 7), and I (day 10)
You should be able to display a schedule similar to the second
illustration with an end date of day 12, and zero slack on all tasks
(no buffers). For reference, the other tasks should start as follows -
day 4 (K), day 5 (J), day 6 (E), day 7 (M, H, B, C), day 9 (G), day 10
(I, L), and day 11 (N). As noted on the web page, this schedule is too
optimistic and delays all tasks to the latest time they can begin.

To introduce the feeding buffers, we will start with task M, which has
two finish to start (FS) constraints from J -> M and K -> M. In the
third and fourth illustrations, a feeding buffer was introduced (the
three small f letters) between tasks K and M. There is an
inconsistency between the third and fourth illustration with the start
date for task M, I will assume task M should start on day 11 (to match
the fourth illustration). You can introduce the three day feeding
buffer between J and M by:
 - setting the start date of task M to day 11.
 - setting the start date of task F to day 5.
The slack shown by Microsoft project for task J should now be three.
The three day slack represents the three day feeding buffer between
task J and task M. Note there is no feeding buffer protecting M from
delays in tasks F and K and the slack is still zero.

You set dates on the other tasks in a similar manner to introduce the
remaining feeding buffers (slack in Microsoft Project) between tasks.
To introduce the project buffer, add a milestone task (O) at the
desired end date. Add a finish to start constraint from M -> O. The
slack of task M should be equal to the number of days in the project
buffer.

The result of this plan should be similar to that in the fourth
illustration. The milestone O should be on day 23 (end of day 22). To
verify, the slack should be...
 - task N, 6 days - project buffer
 - task I, 1 day  - feeding buffer
 - task L, 5 days - feeding buffer
 - task J, 3 days - feeding buffer
Other tasks will have slack of zero (no buffers), or a value matching
one of the buffers listed above.

As you work the project and status the schedule, the predicted end
dates of each task will cause the slack to be updated (usually
decrease) and represents the feeding buffers being consumed. The start
date of a task protected by a buffer will not change until the buffer
is completely consumed (slack is zero).

  --Maniac