CRITICAL CHAIN PROJECT MANAGEMENT – A NEW PROJECT MANAGEMENT
PHILOSOPHY FOR MULTI PROJECT ENVIRONMENT
Jagathipriya, Rajendrasingh.P.Byali, M.V.Kannan
Projects Division, PPEG, ISRO Satellite Centre, Bangalore-560017, India
Traditional project management has been concentrating mainly on single project optimization based upon task
dependency constraints. The Critical Chain Project management (CCPM) provides a substantial improvement in
the PM methodology by incorporating resource dependency and buffer management. CCPM improves the
project scheduling by protecting it from common cause variation (uncertainty, multi-tasking). For a multi-
project environment CCPM is best suitable since critical chain is calculated based upon both resource
dependencies and task dependencies and the constraint of multi-tasking is also delt with. Buffer management
enhances measurement and decision making for project control. This paper provides an insight to the CCPM
concept with a practical example of scheduling multiple satellite projects using this technique.
Keywords: Critical Chain, Project Management, Resource Constraint, Multi-tasking, Buffer Management
TRADITIONAL PROJECT MANAGEMENT: The task commences after half the allotted time is
Traditional Project Management concepts over and once the task commences, if a problem is
have been in use since 1950’s. The development of encountered the buffer allotted becomes useless
PERT/CPM provided the initial breakthrough in and the estimate will certainly overrun.
Project planning/scheduling. But over a period of
time certain limitations in this methodology has Task Estimation in PERT/CPM
necessitated to relook at the philosophy behind PM In traditional PM techniques the safety /
techniques. buffer is included in each and every task to take
care of delays/uncertainties in completion of work.
Limitations of PERT/CPM The contributing factors for this are: non-
• Applicable only for Single Project scheduling. availability of resources in time, to overcome
• Buffer built into each individual activity is overkill technological problems encountered, to incorporate
and inefficient. last minute changes in design/configuration etc.
• Strict adherence to task start date and end date For example - When a task estimate is made
will lead to work expanding to fill the time considering the quantum of work it may require 5
available. days. But safety is added to incorporate unplanned
• In a multi project environment collective load on work interruptions, uncertainty etc and the total
each project resource is not assessed, as each task duration becomes 10 days. This estimate is in
project is scheduled individually which ultimately turn used to calculate the Critical Path of the
leads to multitasking. project.
Most of the organizations work in a multi-
Laws of Project Management: project environment. As in our case Project
Generally the following two laws of Directors are responsible for successful completion
project management influence the task duration of the project. Project Directors have a tendency to
estimation and its completion. be demanding. They think that their project is of
highest priority and they want to see frequent
1. Parkinson’s Law: progress in their project. This focus on showing
Work expands to fit the allotted time. If a progress on as many active projects as possible is
task is estimated to take 10 days, it usually the major cause for multi-tasking.
doesn’t take less. This adjustment of effort to fill
the allotted time can come in number of ways. CRITICAL CHAIN:
Most of the time there is little or no reward for Introduced in 1997, by Eliyahu.M.Goldartt
completing individual activities early. If individuals this methodology is based upon insight into human
complete the activities early, they get to do more nature and what happens when a project
work. Most of the people have a tendency to do management discipline is applied to people. 
the work when the activity completion becomes Critical Chain is defined as the longest
critical. This leads to underutilization of the chain of tasks that consider both task
contingency time resulting in completion of most of dependencies and resource dependencies. This is
the work in the later portion of the scheduled different from the definition of the Critical path,
activity time. which is defined as the longest chain of tasks
based upon task dependencies. Critical Chain
2. Student’s Syndrome: recognizes that a delay in resource availability can
Given a project that is composed of tasks delay a schedule just as a delay in dependant
with time buffer, it leads to Students Syndrome. tasks.
Critical Chain Project Management derives
from applying the Theory of Constraints (TOC) to 1. Task Estimation in CCPM
Project Management. TOC says, “Any system must Critical Chain task estimating requires a
have a constraint. Otherwise, its output would change in individual and organizational behavior to
increase without bound, or go to zero.” TOC be effective. The removal of hidden safety in task
identifies the constraint of a project as Critical durations requires establishment of an
Chain or “The sequence of dependent events that organizational culture that removes fear of
prevents the project from completing in a short exposing this safety and removing it from task
interval. Resource dependencies determine the estimates.
critical chain as much as do task dependencies.” It is important to note that safety is not
removed and thrown away. Instead it is pooled as
Relay Race Approach: a project resource as opposed to a hidden task-
Critical Chain utilizes the relay race level resource. This can be achieved by estimating
approach to capitalize on the benefit of early based on following assumptions:
finishes. In the relay race each runner runs his leg • When deriving an estimate it should be
as fast as possible and hands on the baton off to assumed that all material and information
the next runner who is ready to run as soon as the needed for the task is on hand.
preceding runner is finished. Because the runner • It is assumed that uninterrupted focus on the
capitalizes on an early finish of the preceding task is given.
runner, a fast leg can offset a slow runner to the • It is also assumed that there won’t be any
benefit of the team. Applying this to the critical surprises that cause additional work.
chain project when one task is getting close to
competition, you must have the next task’s 2. As Late As Possible Scheduling:
resource on the track and be ready to start as- The tasks are scheduled as-late-as-
soon-as-possible after the preceding task possible (ALAP) based upon the target end date.
completes. 1. Using production analogy, the Work In
This relay race approach means that the Progress (WIP) is minimized and not incurring
team must de-emphasize the task scheduled start costs earlier than necessary.
and finish dates and instead concentrate on 2. From project executive’s viewpoint, there is
triggering their preparation and start on the better focus at the critical start of the project
preceding task’s progress. Importantly, once a because there simply aren’t many tasks
task is started, people should work as fast as scheduled to start.
possible towards completion without clocking 3. Importantly in complex, knowledge work, your
themselves to the scheduled finish date (As knowledge increases the further you go into
against students’ syndrome). the project. By scheduling tasks ALAP, you are
capitalizing on this increasing knowledge and
The following table gives a general comparison will significantly minimize the need for re-
between Critical Path method and Critical Chain work.
method. 4. If a task is finished earlier than planned, the
Critical Path Method Critical Chain Method project executive might be accused of
Focus is on single Address multi-project sandbagging the estimates instead of being
project scenario as well as rewarded for completing ahead of schedule. In
single project case this environment, the project executive
Focus is directed to Focus is on both worries about the future estimates being cut
manage activities in resource and activity based upon history. Hence he always tries to
critical path – the constraints officially finish on schedule. If the task is
longest path in network finished early then the next problem is
The single project Having a multi project encountered. The task that is dependent upon
optimization leads to environment CC helps the completed task completion might not be
local, rather than global in achieving global able to start early because the required
optimization in multi- efficiency resources are off doing something else.
Slack available in Project buffer is treated 3. Buffer Management:
project is not pooled as project resource and The major difference in Critical Chain is
together and is not buffer management is the pooling of buffer and recognizing it as a project
managed as a resource used as a tool for resource. These buffers automatically contract
project evaluation. when they are pushed by overrunning tasks and
Focus is on individual Project due date is absorb the overruns without affecting the target
due dates of each tasks given importance and is end date.
achieved by monitoring Three types of Buffer have been
the project buffer identified.
- Project Buffer
SALIENT FEATURES OF CCPM - Feeding Buffer
The three salient features of CCPM have been - Capacity Buffer
3.1 Project Buffer: It protects the target end 3.3 Capacity Buffer: Capacity buffer comes into
date against overruns in critical chain tasks. It is picture for multi-project scheduling where the
placed at the end of the project after the last most critical / overloaded resource (known as
Critical Chain task. In effect, the total safety Drum Resource) has to be protected / leveled to
hidden in each individual Critical Chain task is overcome the slippage in schedule due to the non-
reduced and placed in project buffer. availability of drum resource. Capacity Buffer will
be inserted between the last use of the drum
3.2 Feeding Buffer: The Project Buffer protects resource of one project and the first use of the
critical chain from overruns. However the critical drum resource in the next project. This acts as a
chain is exposed to overruns from non-critical protection for the second project against changes
chain tasks that link to the critical chain. Here the in first project that delay the availability of drum
Feeding Buffer, placed at the point where the resource.
feeding chain (non-critical activities) intersects
with the critical chain, protects the critical chain
from the overruns of the non-critical chain tasks.
Fig1: Activity scheduling as in Critical Path
Fig2: Activity scheduling following Critical
The above 2-activity network diagrams provide a the most demand or is so limited by capital costs
simple example to understand the difference that it is the bottleneck around which schedules
between the CPM & CCPM. The resource for each are drawn. There can be more than one drum
activity has been given beside each activity bar resource.
chart. In CPM the activities in red color indicates
the Critical Path. It can be seen that the project Case Study: A practical example has been given
takes 65 days to be completed. It can be noted in below taking into consideration two communication
Fig 2 that each task duration has been reduced by satellites of I-2K bus, which are similar in
50%. Feeding Buffer has been introduced where configuration.
the non-critical chain activities intersect the critical A typical communication satellite takes
chain path. Project Buffer, which is 50% of the 18-24 months for realization (from the
duration of the total Critical Chain activities, has conceptualization to launch) depending on the
been introduced in the end of the project. It can be technical complexities involved. The time taken for
observed that the Project duration has come down integration, from the delivery of structure to AIT
by 10 days (15%). till Shipment of spacecraft for launch is around 11
to 12 months.
Multi Project Critical Chain: From the past experience it is observed
The term Drum Resource refers to the that when a single project team is given the
key, critical resource around which project will be responsibility of realization of projects, which are
synchronized. In production system the drum is similar in configuration, the work will be taken up
the constraint of the system. This constraint sets serially. In other words, the activities related to
the pace of the whole production system just as a one project are completed and then the next
drum is used to set the marching pace of a troop. project activities are taken up. The time taken for
In the project management area, the drum launching the repeat satellite is generally 12 – 14
resource is usually the resource that is always in months from the date of launch of the first
satellite. Considering this an example of 2 Based on the analysis carried out the following
communication satellites has been taken for observations are made:
scheduling using the critical chain method. 1. The time taken for realization of the satellite
In the example considered the activities in the current scenario is around 327 days
from the delivery of structure to AIT has been from the date of delivery of the structure.
considered. Only major milestone activities Using CCPM the duration for each activity has
concerned with delivery of subsystems to AIT, been reduced by 50%. The duration for HILS
integration and testing activities till shipment of test, Thermovac Test and Assembled mode
the satellite for launch is considered. Fig-3 gives Integrated Spacecraft tests are not reduced
the activities scheduled in the Critical exactly by 50% but only the setup time for the
Fig 3: Project scheduling using conventional facility has been reduced. This is because the
method. test duration for these are fixed which is also
dependent on the number of test cycles /
Path method. Fig-4 gives the Schedule for two cases it has to undergo.
Satellite projects planned based on Critical Chain 2. The integration activity of the bus system
method. packages commences as and when the
The following assumptions are made for arriving at packages are delivered to AIT.
the schedule: 3. Thermovaccum facility can be identified as the
• Subsystem configuration of both the satellites Drum resource. This is because being a critical
considered is assumed to be similar. facility it can lead to single point failure if
• The duration time given in Fig-3 is based on advanced planning is not done. But since
previous experience and is being used in the adequate gap is available in the second
organization for scheduling all the spacecraft project for the commencement of the
project activities. Thermovac test Capacity Buffer has not been
• The Relay race approach is assumed: i.e the added.
necessary resources to commence the activity 4. It can be observed that the duration for
as soon as the preceding activity is completed completion of Satellite-1 has been reduced
are available. from 327 days to 231 days i.e by 110 days
(29.35%). Satellite-2 has taken 242 days.
Fig 4: Multiple Satellite Project scheduling delay in the completion of the project
using CCPM activities.
5. The increase of 11 days in Satellite-2 7. When the subsystem realization and
completion is because RCS integration activity integration activities for both the project are
in the Satellite-2 commences only after the taken up in parallel it can be observed that
completion of RCS integration activity of both the satellites can be delivered for launch
Satellite-1. This is assuming that same team is at a gap of 25 days.
working on the integration of RCS for both the
projects. Drawbacks / Limitations of CCPM:
6. The buffer in each activity is made zero and There is no clear-cut answer on the ‘best’
the Project buffer is added at the end for each way to define the critical chain, and often refer to
project (75 days – which is 50% of the total using the strategic considerations when
individual activity buffer). The consumption of determining it. This is similar to the approach
this buffer gives an indication of the extent of taken by Goldartt himself in the development of
the TOC when he declared that a process
bottleneck (or constraint) must be a strategic
decision and not to be based on the accidental References
situation of a plant at a given time.  Goldratt Eliyahu.M., “Critical Chain”, North
The available literature of CCPM does not River, Great Barrington, MA, 1997.
make a distinction between the technological  Goldratt Eliyahu.M., “The Goal”, North River
uncertainty of an activity and the possible delay Press, Croton-on Hudson, New York, 1984.
due to unavailability of the required resources at  Goldratt Eliyahu.M.,”Theory of Constraints”,
the time the activity is due to start. the North river Press, Great Barrington, 1999.
The assumption of multi tasking by  Larry.P.Leach, “Critical Chain Project
resources in CCPM is critical to the roadrunner Management Improves Project Performance”,
mentality of resources, that is, start an activity as Advanced Project Institute, 1997.
soon as the predecessor activities are finished.  Thomas Lechler, Boaz Roven,
However this assumption is generally valid only for Edward.A.Stohar, “Critical Chain: A New Project
production line concept or for projects that are Management Paradigm or Old Wine in New Bottle?”
repetitive in nature. Generally in an R & D Version6/7/05, CPMR, 2005.
organization multi tasking is quite common and  Jaume Ribera, Marc Sachon, Alex Grasas,
cannot be avoided. This may be due to large “Putting the Core Elements of Critical Chain Project
period of idleness that would occur otherwise due Management into perspective: A general
to dependency on external resources for framework for Buffer Management”, IESE Business
materials/components, technological problems, School, Spain, 2003.
delay in finalization of configurations due to multi-
disciplinary interactions etc.
The other critical factor on the roadrunner
mentality is the sharing of information between
activity owners, implemented in CCPM via the
capacity buffers, notifying the delay in availability
of the drum resource fro the next connected
activity. To achieve this, high transparency is
needed on how time is used in the project
In spite of the above limitations Critical
Chain method is becoming popular in Aerospace
industries and is already successfully implemented
in the Software industry for execution of projects.
This paper gives an overview of the CCPM and the
implementation of this method for execution of
spacecraft projects. Also a case study has been
given where in the methodology of implementation
has been discussed. The essential changes
introduced by CCPM can be summarized as
1. Focus on total systems throughput rather
than individual projects.
2. Identify /manage constraint resource to
avoid or solve resource conflicts.
3. Using buffer management as the primary
tool for project management and control.
4. Avoid multitasking which leads to
prioritizing project activities.
To exploit the benefits of this method it is
needed to establish a total project environment
that integrates both the human behavioral
elements and methods into an effective operating
unit. The human side requires everyone, from
management to the project team, to understand
and buy-in to the concepts.
We acknowledge the valuable suggestions given by
Mr.H. Bhojraj, GD PPEG. We also express our
sincere gratitude to Dr.K.N.Shankara, Director
ISAC for his constant encouragement.