The to address these problems and with a goal

The usage of traditional
project management concepts resulted in overrunning of cost and time of the
original estimates. The critical-path based project management was introduced
to address these problems and with a goal to deliver the projects within the
original cost and time estimates. Then in 1997, Dr. Eliyahu M. Goldratt
published a book named “Critical Chain”, the book had information about the
critical chain project management which was based on the principle that every
system has a constraint, and the system performance can only be improved by
enhancing the performance of the constraining resource. CCPM is basically an
extension of theory of constraints.

 

The starting point in CCPM is a list of
tasks with their duration estimates and dependencies. The first step here is
the development of the initial schedule for project tasks and also their
dependencies are taken into account and the availability of resources because
some of the resources have limited availability. The resulting schedule will be
longer with basic critical path algorithm, the critical activities would be
delayed while waiting for the resources. The CCPM identifies the critical chain
as the set of task that result in longest path of project completion after
levelling. The critical chain yields the expected project completion date and
the resources required by the task in the critical chain are defined as the
critical resources. The next step that we have in CCPM planning is the
recalculating of the project schedule based on the original shortened task duration
estimate. There are certain things that we need to understand while shortening
the original duration estimates are as follows:

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Ø  All the tasks in the projects are subject to some degree of
uncertainty.

Ø  When asked to provide an estimate of the duration, the task owner
provides a safety margin in order to be really sure that the task will be
completed in time i.e. the task is overestimated.

Ø  The safety margin is internal to the task; if it is not needed then
it is wasted. The resource for the next task is not available until the
schedule time. Therefore, when it becomes obvious that the buffer is not
required, the task owner will use the buffer time anyway because there is
little or no incentive to finish early. On the other hand any delays to the
task in the critical chain will propagate to the successor tasks. CCPM states
that when original duration estimates are such that the likelihood of
completion is 95% and they should be reduced to 50%. The difference the project
duration based on the new estimates and the original project duration is called
the project buffer and should be displayed in the project Gantt chat as a
separate task. The following is a figure which illustrates the relationships
between the original schedule and the CCPM schedule based on the shortened task
duration.

 

Ø  Fig-8. Conventional Schedule and CCPM Schedule. (CCPM: TOC based Project
Management Technique by Prof. P.M. Chawan, Ganesh P. Gaikwad, Prashant S.
Gosavi; 2012)

 

The
buffers which were previously hidden in each task have been made explicit and
pooled. This pooled buffer is called as the project buffer.  The probability of all the critical chain
tasks exceeding their likelihood completion estimate which is 50% is very less
likely. Under the assumption of the statistical independence, about half of the
task will exceed 50% mark, while the other half will complete the task within
50%. By pooling the safety margins of individual tasks, the protection against
uncertainty is improved. Hence the CCPM suggests that the combined project
buffer should be less than the sum of the safety margins of the individual
activities. This agreement is suggested by the statistical theory which was
previously discussed.

The
same process of making safety margins explicit and pooling them can also be applied
to non-critical paths. As before the safety margin in each task are identified,
taken out and pooled at the end of the path. Because this buffer is placed
where the path feeds back into the critical chain path, it is called a feeding
buffer.  The third type of buffer used by
CCPM is called as resource buffer, which is a virtual task inserted prior to
the critical chain tasks that require critical resources. The main purpose of
doing this is that to issue a signal to the critical resources that a critical
chain task to which they are assigned is due to start.

Ø  Resources working in the
critical chain tasks are expected to work continuously on a single task at a
time. They do not work on several tasks on parallel or suspend their critical
task to do other work.

Ø  If a task is completed
ahead of schedule, work on its successor is to begin immediately. If the task
successor utilizes a critical resource  
for which a resource buffer has been 
defined, advance warning has been provided to the resource at the point
in time where  resource buffer begins;

Ø  If the task is completed
past its planned completion date, as shown on the CCPM schedule, this is no
reason for immediate concern.   As
progress is reported, the CCPM schedule is recalculated, the final due date is
kept constant by dusting the buffer sizes. The project control focuses on the
consumption of the buffer. The buffer consumption is a clear indication that
the corrective actions are required, so the reassessment of the allocation of
resources to the tasks in chain should be done. In this way the buffer
utilization helps us to monitor the likelihood of completion of the project by
its committed due date.Critical Chain Project Management

The usage of traditional
project management concepts resulted in overrunning of cost and time of the
original estimates. The critical-path based project management was introduced
to address these problems and with a goal to deliver the projects within the
original cost and time estimates. Then in 1997, Dr. Eliyahu M. Goldratt
published a book named “Critical Chain”, the book had information about the
critical chain project management which was based on the principle that every
system has a constraint, and the system performance can only be improved by
enhancing the performance of the constraining resource. CCPM is basically an
extension of theory of constraints.

 

The starting point in CCPM is a list of
tasks with their duration estimates and dependencies. The first step here is
the development of the initial schedule for project tasks and also their
dependencies are taken into account and the availability of resources because
some of the resources have limited availability. The resulting schedule will be
longer with basic critical path algorithm, the critical activities would be
delayed while waiting for the resources. The CCPM identifies the critical chain
as the set of task that result in longest path of project completion after
levelling. The critical chain yields the expected project completion date and
the resources required by the task in the critical chain are defined as the
critical resources. The next step that we have in CCPM planning is the
recalculating of the project schedule based on the original shortened task duration
estimate. There are certain things that we need to understand while shortening
the original duration estimates are as follows:

 

Ø  All the tasks in the projects are subject to some degree of
uncertainty.

Ø  When asked to provide an estimate of the duration, the task owner
provides a safety margin in order to be really sure that the task will be
completed in time i.e. the task is overestimated.

Ø  The safety margin is internal to the task; if it is not needed then
it is wasted. The resource for the next task is not available until the
schedule time. Therefore, when it becomes obvious that the buffer is not
required, the task owner will use the buffer time anyway because there is
little or no incentive to finish early. On the other hand any delays to the
task in the critical chain will propagate to the successor tasks. CCPM states
that when original duration estimates are such that the likelihood of
completion is 95% and they should be reduced to 50%. The difference the project
duration based on the new estimates and the original project duration is called
the project buffer and should be displayed in the project Gantt chat as a
separate task. The following is a figure which illustrates the relationships
between the original schedule and the CCPM schedule based on the shortened task
duration.

 

Ø  Fig-8. Conventional Schedule and CCPM Schedule. (CCPM: TOC based Project
Management Technique by Prof. P.M. Chawan, Ganesh P. Gaikwad, Prashant S.
Gosavi; 2012)

 

The
buffers which were previously hidden in each task have been made explicit and
pooled. This pooled buffer is called as the project buffer.  The probability of all the critical chain
tasks exceeding their likelihood completion estimate which is 50% is very less
likely. Under the assumption of the statistical independence, about half of the
task will exceed 50% mark, while the other half will complete the task within
50%. By pooling the safety margins of individual tasks, the protection against
uncertainty is improved. Hence the CCPM suggests that the combined project
buffer should be less than the sum of the safety margins of the individual
activities. This agreement is suggested by the statistical theory which was
previously discussed.

The
same process of making safety margins explicit and pooling them can also be applied
to non-critical paths. As before the safety margin in each task are identified,
taken out and pooled at the end of the path. Because this buffer is placed
where the path feeds back into the critical chain path, it is called a feeding
buffer.  The third type of buffer used by
CCPM is called as resource buffer, which is a virtual task inserted prior to
the critical chain tasks that require critical resources. The main purpose of
doing this is that to issue a signal to the critical resources that a critical
chain task to which they are assigned is due to start.

Ø  Resources working in the
critical chain tasks are expected to work continuously on a single task at a
time. They do not work on several tasks on parallel or suspend their critical
task to do other work.

Ø  If a task is completed
ahead of schedule, work on its successor is to begin immediately. If the task
successor utilizes a critical resource  
for which a resource buffer has been 
defined, advance warning has been provided to the resource at the point
in time where  resource buffer begins;

Ø  If the task is completed
past its planned completion date, as shown on the CCPM schedule, this is no
reason for immediate concern.   As
progress is reported, the CCPM schedule is recalculated, the final due date is
kept constant by dusting the buffer sizes. The project control focuses on the
consumption of the buffer. The buffer consumption is a clear indication that
the corrective actions are required, so the reassessment of the allocation of
resources to the tasks in chain should be done. In this way the buffer
utilization helps us to monitor the likelihood of completion of the project by
its committed due date. 

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